Of course Apple will take a hit when Steve steps down, whether because he is ill, or because he retires, or whatever. He is the personal embodiment of much of Apple's luster. It does not require a crystal ball to see that.
Nevertheless, he's not an idiot, and Apple's board is not composed of idiots.
I have no worries about them having a plan already in place, with some number of contingencies. Apple will suffer briefly for whatever takes Steve out, but they will bounce back fairly quickly. He's not irreplaceable.
Look for someone helping him at the next big show. They'll bring about this change in stages.
Alighting upon the obvious with a sense of discovery.
Isn't Al Gore on the board of Apple?