Someone joked on Page 1 (too lazy to go back and quote them) that Steve couldn't possibly talk 90 minutes about Leopard.... Well Steve DID fill up 90 minutes talking about a frickin cell phone in January.
But honestly I would be surprised to see ANY machine updates at WWDC. It's a DEVELOPER's conference. That mainly entails software and what other than Operating Systems do developers need to be told about?
So being that it is a software show and also a show that happens to be right before the iPhone launch, here are my predictions for WWDC this year:
Steve will discuss the iTunes Store (Plus, movies, shows, market share, etc...)
Then he will lead into the

tv (Discussing the new model)
Steve will shift gears and discuss the iPhone and all of its features that he couldn't go over at MacWorld.
Segue into Leopard. This will take the bulk of the time, maybe 45-50 minutes worth.
After Leopard, we will get to see the iLife collection which will now be called iHome. Just my stupid guess on the name, because....
... the brand new iWork will be unveiled.
Both iApp Suites will heavily incorporate Leopard's new secret features.
End with some "little heard from" singer playing the guitar and singing his new single that's available in the iTunes Plus Store as of today.