...they have had several of these now and Cook has stood in brilliantly in all of them. Over the last year or two they have pushed more and more people to the front of the presentations... Check out the "Back to the mac" event and you can see, Steve is more or less the MC... They've known this for a long time...
Then you have Tim Cook on stage with Verizon instead of Steve... It's not a sudden thing, they are managing the message, like they do with EVERYTHING apple makes, says or does...
And let's be honest, Tim Cook has been taking care of "day to day operations" at Apple for the last 10 years. Steve is the vision guy who happens to be fanatical about the execution of that vision down the last detail. But he's spent 10 years getting others to be equally fanatical.
He has taken an almost bankrupt company to be the second most valuable in the world (who knows when it will become the MOST valuable, I wouldn't write that off, given where they started in 1997 when he returned!)
If I were Jobs, I'd say "I think I did what I was asked to do and I think I can retire now." But he's Jobs, so he will forever be part of apple product strategy until the day he dies (which is hopefully many many years away.) Hence, they are managing the message so they wind up with a new CEO but Jobs very much seen as the man with the vision... They'll wind that bit down over time and ramp Ives, Schiller etc up on that side...
They are a smart group of business people and they are figuring out how to manage the (private) issue of a CEOs health against their duty to their shareholders.
No one can argue that Apple hasn't delivered for it's shareholders...
Just my 2c/p.