For some time I have been using the free TCMP player (I just searched google for it and still find it for download on the net) and pocket divx converter to convert mpg's i had downloaded. The quality isn't so hot, but I am using an Axim X5 with ppc2003 & low ram, etc., so...
I usually just listen to it when I drive, not try to watch it, so it has worked for me.
Someone recently told me about Slysoft's AnyDVD, CloneDVD, and their mobile apps.
www.slysoft.com. They are for purchase, and offer 15 day trials. I have successfully copied commercial dvds, episodes off episodic dvds, and crunched them down and played them on my axim with these softwares.
Haven't tried the spb one mentioned here. Does it pull video off commercial dvd's for playback on ppc?