I actually haven't watched all of Apple's keynotes, but from what I have gathered, my understanding is that Tim Cook doesn't have the same great speaking style that Steve Jobs has. In fact, Tim's a quiet one. So, maybe he won't be presenting at all? He might just leave it to the VP's to present, like how Scott Forstall presents the iOS updates, and how a different VP for Lion presented at the WWDC. So maybe, we might get the designer to present the hardware portion, and Scott Forstall to present the software features (assuming there will be new features like Nuance).
Or... praying and hoping here... Steve Jobs will be there at least for one bit. But personally... I can't imagine Tim Cook as the main presenter. He just doesn't have the same aura as Steve Jobs.