hola,
mencoder input.<whatever> -o output.mov -nosound -ovc lavc -lavcopts vcodec=mjpeg -of lavf -lavfopts format=mov
and if you want to play or see youtube videos ( specially thinking here of people who don't have the flash plugin, like those using linux on a ppc ) :
download http://www.arrakis.es/~rggi3/youtube-dl/
and :
youtube-dl http://www.youtube.com/watch?v=AGPZY3GhtWA mencoder AGPZY3GhtWA.flv -o AGPZY3GhtWA.mov -oac pcm -ovc lavc -lavcopts vcodec=mjpeg -of lavf -lavfopts format=mov -lavfopts i_certify_that_my_video_stream_does_not_use_b_frames
saludos, sevy