For another thing, Apple's executive bench is deeper than it looks; COO Tim Cook is not new to the role of understudy to Jobs, having filled in during Jobs' first two leaves. It's hard to tell whether he has the same ability to drive and inspire Apple's designers and engineers for which Jobs is famous, and that's the part that could prove tricky. Given that the iPhone 5 and iPad 2 are headed into manufacturing, any problems with those won't belong to Cook. But if Jobs' medical leave extends into 2012, Cook will be the one credited with the success or failure of the devices that follow those models.