Rockbox does not play videos put on your iPod by itunes encoded in the .mp4 format.
itunes puts your videos somewhere in the iPod_control folder, but there's no way of knowing which file it is beneath the obfuscated file structure.
Rockbox can play videos though the playback is the worst on the iPod video, due to its larger screen and the fact that rockbox does not use the 5g's video chip (because there is no public docs for the devs to start work on it)
If you have a nano, video playback should be fine in Rockbox, if you own a 5g I found that the only watchable thing is south park, or similar low visual quality video at low bitrates. If you have a 5g I'd recommend booting into the apple OS to view your videos.
still interested about video in rockbox?
http://www.rockbox.org/twiki/bin/view/Main/PluginMpegplayer