1. Steve loves Apple, and I can't imagine him wanting to leave any time soon
2. Apple has a lot of momentum right now, but I think they would instantly lose much of that momentum if Steve were to leave right now. I'm not saying the company would instantly go to hell in a hand basket, but the company would have less of a cult following, and without Steve's keynotes there wouldn't be as much press coverage of Apple.
3. I don't think Steve is out of ideas just yet.
If I were to guess, I would say in roughly 8 years Apple will introduce the next gen operating system (OS11, though i'm sure that's not what it will be called), and that's when Steve will start to transition his way out of Apple, much the way Bill Gates is doing now with Vista about to come out.