New firmware files in 3.0. (iPhone 1,3)
Photo's revealing "Video Upload"
Photo's released today on video recording
The massive rush that AT&T is going through to update their services before June
Apple says yearly refresh
Theres 5. All of which can be found on Macrumors.
Actually, while I think there will be new hardware, none of the above reasons mean there will be new hardware. Video recording can be performed on current 3G (via jailbreak), I don't see where AT&T is rushing to do anything but take our money, Apple does not say yearly "refresh" - they said the "product cycle" is in June/July.
2 better reasons there will be new hardware are: (1) new hardware means more profit for Apple and (2) carriers (like O2) trying to sell off their existing stock of the 3Gs.
Anyways, I don't think Jobs will be doing the keynote, but I don't think it matters. I think the only piece of "evidence" we have of his health is his missed Disney shareholders meeting. His official word was that he was temporarily stepping down from being the CEO and daily decision maker of Apple, not dropping out of life.