It seems there are one of two possibilities:
1) They have something really big to announce tomorrow and they want to really hype it up
2) They want to demo something that requires WiFi and don't want the entire WiFi system to be brought down by hundreds of bloggers
I'm guessing (2) could be averted by setting up a private WiFi network with security enabled -- so here is hoping for #1.
Also, I'm curious if for some reason Safari running on a PC will not be able to support the live video stream? Or for that matter, do other browsers not yet support this open standard for Live Streaming over HTTP? It could be they are saying Mac, iPhone or iPad just to tout their products, but I can't imagine a user-agent check being used when they so proudly say "Open".