$ ./ffmpeg.exe -i redcliff.webm -c:copy out.mp4
ffmpeg version N-41027-ge02e58f Copyright (c) 2000-2012 the FFmpeg developers
  built on May 25 2012 13:09:41 with gcc 4.6.3
  configuration: --enable-gpl --enable-version3 --disable-w32threads --enable-runtime-cpudetect --enable-avisynth --enable-bzlib --enable-frei0r --enable-libass --enable-libcelt --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libfreetype --enable-libgsm --enable-libmp3lame --enable-libnut --enable-libopenjpeg --enable-librtmp --enable-libschroedinger --enable-libspeex --enable-libtheora --enable-libutvideo --enable-libvo-aacenc --enable-libvo-amrwbenc --enable-libvorbis --enable-libvpx --enable-libx264 --enable-libxavs --enable-libxvid --enable-zlib
  libavutil      51. 54.100 / 51. 54.100
  libavcodec     54. 23.100 / 54. 23.100
  libavformat    54.  6.100 / 54.  6.100
  libavdevice    54.  0.100 / 54.  0.100
  libavfilter     2. 77.100 /  2. 77.100
  libswscale      2.  1.100 /  2.  1.100
  libswresample   0. 15.100 /  0. 15.100
  libpostproc    52.  0.100 / 52.  0.100
Input #0, matroska,webm, from 'redcliff.webm':
  Duration: 00:02:14.09, start: 0.000000, bitrate: 733 kb/s
    Stream #0:0(eng): Video: vp8, yuv420p, 450x256, SAR 1:1 DAR 225:128, 30 fps, 30 tbr, 1k tbn, 1k tbc (default)
    Metadata:
      title           : VP8 video stream
    Stream #0:1(eng): Audio: vorbis, 22050 Hz, stereo, s16 (default)
At least one output file must be specified

$ ./ffmpeg.exe -i redcliff.webm -c:copy -o out.mp4
ffmpeg version N-41027-ge02e58f Copyright (c) 2000-2012 the FFmpeg developers
  built on May 25 2012 13:09:41 with gcc 4.6.3
  configuration: --enable-gpl --enable-version3 --disable-w32threads --enable-runtime-cpudetect --enable-avisynth --enable-bzlib --enable-frei0r --enable-libass --enable-libcelt --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libfreetype --enable-libgsm --enable-libmp3lame --enable-libnut --enable-libopenjpeg --enable-librtmp --enable-libschroedinger --enable-libspeex --enable-libtheora --enable-libutvideo --enable-libvo-aacenc --enable-libvo-amrwbenc --enable-libvorbis --enable-libvpx --enable-libx264 --enable-libxavs --enable-libxvid --enable-zlib
  libavutil      51. 54.100 / 51. 54.100
  libavcodec     54. 23.100 / 54. 23.100
  libavformat    54.  6.100 / 54.  6.100
  libavdevice    54.  0.100 / 54.  0.100
  libavfilter     2. 77.100 /  2. 77.100
  libswscale      2.  1.100 /  2.  1.100
  libswresample   0. 15.100 /  0. 15.100
  libpostproc    52.  0.100 / 52.  0.100
Input #0, matroska,webm, from 'redcliff.webm':
  Duration: 00:02:14.09, start: 0.000000, bitrate: 733 kb/s
    Stream #0:0(eng): Video: vp8, yuv420p, 450x256, SAR 1:1 DAR 225:128, 30 fps, 30 tbr, 1k tbn, 1k tbc (default)
    Metadata:
      title           : VP8 video stream
    Stream #0:1(eng): Audio: vorbis, 22050 Hz, stereo, s16 (default)
[mp4 @ 00000000003331a0] Invalid stream specifier: copy.