the newest divx is based on MPEG4, but has its differences (some would say enhancements, but since we've never seen a true implementation of MPEG4, who knows?..)
however, most divx files on the internet are encoded with the old divx.. this is a completely different codec, and actually simply a hack of a proprietary microsoft codec. for this to work correctly in quicktime, quicktime needs to be changed to be able to read avi's the same way Windows media players read them.
my point is, MPEG4 and the ability to watch divx files on your mac are two very different things. we will be seeing MPEG4 tomorrow.. it'd be great to see them change quicktime to be able to play divx files too, but i doubt they'll do it. apple would much rather people use their own MPEG4 codec, than use divx (a competitor's product). if apple's product can encode faster than divx, divx should die very soon. i can't wait!