I could see Steve stepping down. The way he talked about the iPhone at WWDC seemed so... reverent. (I'm struggling for words, here). He seemed satisfied, not like someone who's finished running a marathon, but someone who was about to see his son graduate from college.
Maybe iPhone is his swan-song. He may keep his feet in the water, be on a few boards, consult privately for $100k an hour, whatever. But I could absolutely see him walk off center stage, not having peaked, but at his peak.