Canon's lineup is diverse enough that a one-size-fits-all Digic V doesn't seem to make sense. Actually I've long been suspicious of the "Digic" label suggesting that in fact all camera Digic chips are the same (there is a variant for semipro and professional camcorders which I think is not part of the discussion, as I don't see canon's DSLR video replacing the functions and specific rationale of camcorders and their processors, though the new DIGICs for still cameras are getting pretty decent in the video area).
If it has actually been the case that there is indeed just a single DIGIC part used for multiple product lines (with perhaps different clock rates dependent on the model of camera) you can likely chalk it up to the costs of custom microprocessor production, or perhaps to ease of designing the models (differentiating by physical features independent of the CPU, like AF points, form factor, and software features changed relatively inexpensively in software - going to avoid opening the AF microadjustment debate again).
But if they have a very powerful 30MP+ camera coming out (and I think it's safe to assume they are at least planning for it in their successor to the Digic IV), trying to create a two-processor system that is still efficient makes less sense than creating a powerful enough single CPU in the first place. With two chips, you don't get the full advantage of the VSLI method - fewer components on the board, resulting in less costs in various steps of production (part production and board assembly, i.e. two CPUs with a given area vs. one chip that will have at least slightly less area than the two combined because of duplicated functions), space saved, and probably lower energy use as well. Right now it seems very wasteful to have two identical chips trading processing tasks (in the 7D, for instance) for burst rates when at least a portion of those chips are redundant - and considering that you now have the added problems of multiprocessor design, not the least splitting memory and the data bus between those processors, a single-chip alternative looks much better. I imagine power management would be slightly simplified too (depends on whether the DIGIC IV is in a ready state soon enough to catch the second frame of a burst for processing from a dormant, unpowered state, but in any case the best desktop CPUs have gotten pretty sophisticated about power usage, and even ~10 year old chips by obscure manufacturers had some power throttling capabilities).
Of course, it remains to be seen if the new CPUs will be fully unlocked on lower-end models - well, that depends on where they put the line for multiple CPUs, but they may still make enough money to justify not using the whole potential of a chip. Plus, there are probably other areas where they would save money on cheap cameras that would legitimately cripple the performance of a DIGIC, like a slower data bus, less memory in the buffer, etc. I'm assuming that compacts have used Digics, and this would suggest that the "little brother" DIGIC would go into cheaper cameras with lower throughput. Or perhaps compacts can stick to Digic IV a little while longer. Lots of questions, no clear answers (it is a rumor, after all).
