Ashley said:
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. The software support DVD playback, so you might be lucky. Another option is software based dvd playback, which doesn't use h/w to render the frames. The reason for this is that h/w implementations often use a layered approach on output to the screen, so what you see is not what is rendered by your window manager.
By default, mplayer uses the XV extention (XVideo which is based on Overlay, IIRC) - although you can configure it to use SDL (-vo sdl) or native xlib (-vo x11) - the SDL option is "grabable" I think, but you will find, but is ALOT slower than the alternatives.
-Darran