Microsoft has confirmed that Halo 3 will have it's video played at E3. This is what Shane Kim, General Manager of Microsoft Game Studios, said in an interview with a New Zealand entertainment website:
"I have nothing to announce now, but expect there to be a significant announcement at E3".

I doubt we'll see anything but a possible demo this year but next year no doubt we should see it around Q1 2007. Long wait for a great game.

Thanks, IceyStopSnitchin211 for the news!