Following the links from Sanmanc I just encoded a full DVD rip which came out at only 481MB. Copied it over to the Pre, opened video's > hit play > working perfectly. At the 450kbps bitrate it looks super crisp on this screen - not showing any artifacts or anything I usually see on Divx/xvid files. The movie I did was "Superbad" (because I already had the vob files on my HDD) which doesn't have much action/fast motion so ymmv.
Thanks for the tips on getting this going, now in handbrake I can just select the Palm Pre settings and convert stuff in no time.
I figure I'll put a couple movies on there for whenever I get bored or to show off the Pre, and maybe a Disney movie to keep the little one busy. You can never underestimate the value of having something to entertain a child at just the right time