in ffserver.conf:
        <Feed stream.ffm>
                File /tmp/stream.ffm
                FileMaxSize 200K
        </Feed>
        <Stream thumb.mjpg>
               Feed stream.ffm
               Format mpjpeg
               VideoFrameRate 15
               VideoIntraOnly
               VideoSize 160x128
               NoAudio
        </Stream>


cmdline:
        # /usr/local/bin/ffmpeg -i socket.mpg http://localhost:8090/stream.ffm

----output----

FFmpeg version SVN-r7225, Copyright (c) 2000-2006 Fabrice Bellard, et al.
  configuration:
  libavutil version: 49.1.0
  libavcodec version: 51.25.0
  libavformat version: 51.6.0
  built on Dec  5 2006 15:53:55, gcc: 4.0.4 20060904 (prerelease) (Debian 4.0.3-7)
Input #0, mpeg, from 'socket.mpg':
  Duration: N/A, bitrate: 2384 kb/s
  Stream #0.0[0x1e0]: Video: mpeg2video, yuv420p, 320x240, 2000 kb/s, 29.97 fps(r)
  Stream #0.1[0x1c0]: Audio: mp2, 48000 Hz, stereo, 384 kb/s
127.0.0.1 - - [Tue Dec  5 16:39:11 2006] "GET /stream.ffm HTTP/1.0" 200 8245
Output #0, ffm, to 'http://localhost:8090/stream.ffm':
  Stream #0.0: Video: mjpeg, yuv420p, 320x240, q=3-31, 64 kb/s, 10.00 fps(c)
Stream mapping:
  Stream #0.0 -> #0.0
[mjpeg @ 0x84118dc]colorspace not supported in jpeg
Error while opening codec for output stream #0.0 - maybe incorrect parameters such as bit_rate, rate, width or height