On a more serious note, I think he’s not showing up because of his health. Investor confidence (and Apples stock price) would not take kindly to seeing a sickly Jobs at the expo. Rightly or wrongly, Apple is seen as very dependant on Jobs and if people think he’s not long for this world it will hurt the company greatly.
It’s more simple. The company cannot be run on a cult of personality forever. Jobs will slowly withdraw his publicity to that of a regular CEO so his retirement will not create a public shock. Given that, I predict retirement within five years, since that would make for a nice, easy transition.