Yeah, when Apple ships something in September the press will say "I guess they can survive without Steve" even though those products have been ready to go for a while.
It could actually be that Steve's health has not changed radically but it's a move he needed to make sooner or later and he has huge confidence in the upcoming product cycle to cement Tim's new role.
I don't typically like to spread rumors, but I work as a bartender in Walnut Creek, CA and a guest that claimed to have close mutual friends with Steve said it was defintely because of his health. I obviously can't substantiate her claim, but she seemed extremely sad and elaborated quite a bit.
I definitely hope she is wrong and wish him the best.
@Steko: I saw a supposedly new, and saddening, photo of Steve Jobs in which he looks emaciated. Maybe it is the way he is dressed that makes him look so much worse than what we are used to.
