Wirelessly posted (Mozilla/5.0 (iPod; U; CPU iPhone OS 4_3_5 like Mac OS X; en-us) AppleWebKit/533.17.9 (KHTML, like Gecko) Version/5.0.2 Mobile/8L1 Safari/6533.18.5)
Seems to me Steve will be on stage one way or another at some point during this keynote.
Nailed on IMHO, all fits
If the keynote was taking place in downtown San Francisco or whatever then Steve has to drive or be driven down there, likely in heavy traffic, tiring for a man who is not very well
He would already be on campus doing his chairman's job and holding meetings etc so it easy for him just to pop into the auditorium, less tiring, less effort for him, smaller stage
I am 99.99% certain we will see Steve playing an active role in the launch