The rev A PBG4's have on-chip cache (256kB?) as well as the 1MB @ 1/2clock off-chip cache.
The rev B just has the on-chip (512kB) cache.
As previously mentioned, the rev A has a much wimpier graphics card.
The rev B comes with CD-RW/DVD combo drive while rev A only has CD-ROM/DVD drive. According to Apple, the rev A's don't have enough space for the new burner combo drive.
The rev A 500 & rev B 550 have the same size HD stock (20GB), but this is upgradeable.
The rev B comes with a more powerful battery (55W/hrs vs. 50W/hrs) don't know how much of a difference that is, but the new batteries do work on the rev A PB's.
Internals are different in structure. The rev B PB's have a different heat sink setup that's supposedly better.
I can't think of anything else at the moment. Hope this helps!