My filter:
"filters": {
       "foo": "fun({Doc},Req) -> true end."
   }
   
Error report:
1> [info] [<0.22796.0>] 127.0.0.1 - - 'GET' /mytemp/_changes?filter=erl/foo 200
1> [error] [<0.22800.0>] ** Generic server <0.22800.0> terminating 
** Last message in was {prompt,
                           [<<"ddoc">>,<<"_design/erl">>,
                            [<<"filters">>,<<"foo">>],
                            [[{[{<<"_id">>,
                                 <<"854d5890751755bddd7c7d3b58000dcc">>},
                                {<<"_rev">>,
                                 <<"1-cd90201763f897aa0178b7ff05eb80cb">>},
                                {<<"x">>,1},
                                {<<"y">>,2},
                                {<<"_revisions">>,
                                 {[{<<"start">>,1},
                                   {<<"ids">>,
                                    [<<"cd90201763f897aa0178b7ff05eb80cb">>]}]}}]}],
                             {[{<<"info">>,
                                {[{db_name,<<"mytemp">>},
                                  {doc_count,19},
                                  {doc_del_count,0},
                                  {update_seq,46},
                                  {purge_seq,0},
                                  {compact_running,false},
                                  {disk_size,208987},
                                  {instance_start_time,<<"1271350757823718">>},
                                  {disk_format_version,5}]}},
                               {<<"id">>,null},
                               {<<"method">>,'GET'},
                               {<<"path">>,[<<"mytemp">>,<<"_changes">>]},
                               {<<"query">>,{[{<<"filter">>,<<"erl/foo">>}]}},
                               {<<"headers">>,
                                {[{<<"Accept">>,<<"*/*">>},
                                  {<<"Host">>,<<"localhost:5984">>},
                                  {<<"User-Agent">>,
                                   <<"curl/7.19.7 (universal-apple-darwin10.0) libcurl/7.19.7 OpenSSL/0.9.8l zlib/1.2.3">>}]}},
                               {<<"body">>,undefined},
                               {<<"peer">>,<<"127.0.0.1">>},
                               {<<"form">>,{[]}},
                               {<<"cookie">>,{[]}},
                               {<<"userCtx">>,
                                {[{<<"db">>,<<"mytemp">>},
                                  {<<"name">>,null},
                                  {<<"roles">>,[<<"_admin">>]}]}}]},
                             {[{<<"db">>,<<"mytemp">>},
                               {<<"name">>,null},
                               {<<"roles">>,[<<"_admin">>]}]}]]}
** When Server state == {evstate,
                            {dict,1,16,16,8,80,48,
                                {[],[],[],[],[],[],[],[],[],[],[],[],[],[],[],
                                 []},
                                {{[],[],[],[],[],[],[],[],
                                  [[<<"_design/erl">>|
                                    {[{<<"_id">>,<<"_design/erl">>},
                                      {<<"_rev">>,
                                       <<"21-a01ff617a71f03ba67dad709e4737258">>},
                                      {<<"language">>,<<"erlang">>},
                                      {<<"filters">>,
                                       {[{<<"foo">>,
                                          <<"fun({Doc},Req) -> true end.">>}]}}]}]],
                                  [],[],[],[],[],[],[]}}},
                            [],
                            {[{<<"reduce_limit">>,true}]},
                            nil,5000}
** Reason for termination == 
** {function_clause,[{couch_native_process,ddoc,4},
                     {couch_native_process,handle_call,3},
                     {crypto,sha,1}]}

1> 
=ERROR REPORT==== 15-Apr-2010::15:57:51 ===
** Generic server <0.22800.0> terminating 
** Last message in was {prompt,
                           [<<"ddoc">>,<<"_design/erl">>,
                            [<<"filters">>,<<"foo">>],
                            [[{[{<<"_id">>,
                                 <<"854d5890751755bddd7c7d3b58000dcc">>},
                                {<<"_rev">>,
                                 <<"1-cd90201763f897aa0178b7ff05eb80cb">>},
                                {<<"x">>,1},
                                {<<"y">>,2},
                                {<<"_revisions">>,
                                 {[{<<"start">>,1},
                                   {<<"ids">>,
                                    [<<"cd90201763f897aa0178b7ff05eb80cb">>]}]}}]}],
                             {[{<<"info">>,
                                {[{db_name,<<"mytemp">>},
                                  {doc_count,19},
                                  {doc_del_count,0},
                                  {update_seq,46},
                                  {purge_seq,0},
                                  {compact_running,false},
                                  {disk_size,208987},
                                  {instance_start_time,<<"1271350757823718">>},
                                  {disk_format_version,5}]}},
                               {<<"id">>,null},
                               {<<"method">>,'GET'},
                               {<<"path">>,[<<"mytemp">>,<<"_changes">>]},
                               {<<"query">>,{[{<<"filter">>,<<"erl/foo">>}]}},
                               {<<"headers">>,
                                {[{<<"Accept">>,<<"*/*">>},
                                  {<<"Host">>,<<"localhost:5984">>},
                                  {<<"User-Agent">>,
                                   <<"curl/7.19.7 (universal-apple-darwin10.0) libcurl/7.19.7 OpenSSL/0.9.8l zlib/1.2.3">>}]}},
                               {<<"body">>,undefined},
                               {<<"peer">>,<<"127.0.0.1">>},
                               {<<"form">>,{[]}},
                               {<<"cookie">>,{[]}},
                               {<<"userCtx">>,
                                {[{<<"db">>,<<"mytemp">>},
                                  {<<"name">>,null},
                                  {<<"roles">>,[<<"_admin">>]}]}}]},
                             {[{<<"db">>,<<"mytemp">>},
                               {<<"name">>,null},
                               {<<"roles">>,[<<"_admin">>]}]}]]}
** When Server state == {evstate,
                            {dict,1,16,16,8,80,48,
                                {[],[],[],[],[],[],[],[],[],[],[],[],[],[],[],
                                 []},
                                {{[],[],[],[],[],[],[],[],
                                  [[<<"_design/erl">>|
                                    {[{<<"_id">>,<<"_design/erl">>},
                                      {<<"_rev">>,
                                       <<"21-a01ff617a71f03ba67dad709e4737258">>},
                                      {<<"language">>,<<"erlang">>},
                                      {<<"filters">>,
                                       {[{<<"foo">>,
                                          <<"fun({Doc},Req) -> true end.">>}]}}]}]],
                                  [],[],[],[],[],[],[]}}},
                            [],
                            {[{<<"reduce_limit">>,true}]},
                            nil,5000}
** Reason for termination == 
** {function_clause,[{couch_native_process,ddoc,4},
                     {couch_native_process,handle_call,3},
                     {crypto,sha,1}]}
1> [error] [<0.22800.0>] {error_report,<0.34.0>,
    {<0.22800.0>,crash_report,
     [[{initial_call,{couch_native_process,init,['Argument__1']}},
       {pid,<0.22800.0>},
       {registered_name,[]},
       {error_info,
           {exit,
               {function_clause,
                   [{couch_native_process,ddoc,4},
                    {couch_native_process,handle_call,3},
                    {crypto,sha,1}]},
               [{gen_server,terminate,6},{proc_lib,init_p_do_apply,3}]}},
       {ancestors,
           [couch_query_servers,couch_secondary_services,couch_server_sup,
            <0.35.0>]},
       {messages,[]},
       {links,[<0.91.0>,<0.22796.0>]},
       {dictionary,[]},
       {trap_exit,false},
       {status,running},
       {heap_size,987},
       {stack_size,24},
       {reductions,8725}],
      [{neighbour,
           [{pid,<0.22796.0>},
            {registered_name,[]},
            {initial_call,
                {mochiweb_socket_server,acceptor_loop,['Argument__1']}},
            {current_function,{gen,wait_resp_mon,3}},
            {ancestors,
                [couch_httpd,couch_secondary_services,couch_server_sup,
                 <0.35.0>]},
            {messages,[]},
            {links,[<0.97.0>,<0.22800.0>,#Port<0.2438>]},
            {dictionary,
                [{mochiweb_request_body,undefined},
                 {mochiweb_request_qs,[{"filter","erl/foo"}]},
                 {jsonp,no_jsonp},
                 {mochiweb_request_cookie,[]}]},
            {trap_exit,false},
            {status,waiting},
            {heap_size,6765},
            {stack_size,12},
            {reductions,10665}]}]]}}
1> 
=CRASH REPORT==== 15-Apr-2010::15:57:51 ===
  crasher:
    initial call: couch_native_process:init/1
    pid: <0.22800.0>
    registered_name: []
    exception exit: {function_clause,[{couch_native_process,ddoc,4},
                                      {couch_native_process,handle_call,3},
                                      {crypto,sha,1}]}
      in function  gen_server:terminate/6
    ancestors: [couch_query_servers,couch_secondary_services,
                  couch_server_sup,<0.35.0>]
    messages: []
    links: [<0.91.0>,<0.22796.0>]
    dictionary: []
    trap_exit: false
    status: running
    heap_size: 987
    stack_size: 24
    reductions: 8725
  neighbours:
    neighbour: [{pid,<0.22796.0>},
                  {registered_name,[]},
                  {initial_call,
                      {mochiweb_socket_server,acceptor_loop,['Argument__1']}},
                  {current_function,{gen,wait_resp_mon,3}},
                  {ancestors,
                      [couch_httpd,couch_secondary_services,couch_server_sup,
                       <0.35.0>]},
                  {messages,[]},
                  {links,[<0.97.0>,<0.22800.0>,#Port<0.2438>]},
                  {dictionary,
                      [{mochiweb_request_body,undefined},
                       {mochiweb_request_qs,[{"filter","erl/foo"}]},
                       {jsonp,no_jsonp},
                       {mochiweb_request_cookie,[]}]},
                  {trap_exit,false},
                  {status,waiting},
                  {heap_size,6765},
                  {stack_size,12},
                  {reductions,10665}]
1> [error] [<0.97.0>] {error_report,<0.34.0>,
    {<0.97.0>,std_error,
     {mochiweb_socket_server,235,
         {child_error,
             {function_clause,
                 [{couch_native_process,ddoc,4},
                  {couch_native_process,handle_call,3},
                  {crypto,sha,1}]}}}}}
1> 
=ERROR REPORT==== 15-Apr-2010::15:57:51 ===
{mochiweb_socket_server,235,
    {child_error,
        {function_clause,
            [{couch_native_process,ddoc,4},
             {couch_native_process,handle_call,3},
             {crypto,sha,1}]}}}