For the thread corresponding to the subject of this thread (which is more about remux than demux), see here: http://macosx.com/forums/mac-os-x-system-mac-software/273592-how-do-you-unmux-mpeg-file.html
ffmpegX does not want (in the version I have) to demux files which are not mpg. You can demux other with the command line ffmpeg (for instance, unprotected .m4v).
The command lines for demuxing are:
-> to get the audio
ffmpeg -i input -vn -acodec copy output
-> to get the video
ffmpeg -i input...