The big software change
So, I stumbled across this article the other day about Apple's chip roadmap, and it suggests that they might move to 64bit ARM by 2014. Could this be the shift Apple is waiting for to shake up it's operating systems? Conceivably they could merge both at that time, and make a lot of big changes then. Maybe for know they're just focused on iterative updates and refinements on both platforms (on a yearly schedule) because they're waiting?
Or maybe they're just getting bloated. Who knows. This could explain some of their strategy, though!