Try mplayer (www.mplayerhq.hu/homepage/). It allows you to pipe frames to a file. I managed to get a set of screenshots out of a DivX movie with it.
I tried it before- I didn't really like it all that much, I forget the reason why. It's no matter, I just wanted to include a DVD playing on a screenshot as a sort of "this is what Linux can do" propoganda exercise to some friends! ;)
Ricardo