I'm thinking he won't show up. He's not the CEO now, and he's been doing less and less on the keynotes in the past. My suspicion is that they want a clean cutover and it will be all Tim's show.
Doubt it tbh
If Tim does it then the focus will be in the media on Steve not being there and it would be a distraction from the message that Tim is trying to get across, can imagine headlines in the media like
"Fears grow for Steve Jobs as he misses keynote"
Whereas if Steve does the keynote it will be
"Steve Jobs leaves sickbed to launch iPhone5"
So if Steve does the keynote the iPhone 5 would get more attention than if Tim does it and Steve isn't there.