The vast majority of the downsizing is because going from NTSC to PPC resolution is about... 4 times fewer pixels, so "full screen" poceket pc movies, with nothing else changed, are 4 times less.
That means 1 GB can convert to about 320 MB just from that. Converting the audio track can also have a similar effect, by making it a lower bit rate MP3, or some other kind of format. Making it mono makes a huge difference, if you don't plan to use headphones, especially.
Now that I actually just got some cards, I willk be able to try this out myself
Finally, the actual frame rate itself can be slowed, by say 1/3. Combine that with teh cuts to audio, and the conversion to DIVX and you have something that can fit on CF card of some kind...
There's only 1 problem though... DVDs usually aren't just 1 gig...