The reason I put Halo 3 in quotations is because we all know that's bullsh*t. This isn't a teaser for Halo 3. This is a proof of concept for the Halo movie. Putting Halo 3 on it was just a way for them to get it out there.
http://www.youtube.com/watch?v=rG6NHmeIW2A
If a Halo movie doesn't get made now...I'll kill myself.
Edit: 360 owners can also download it in 480p or 720p for HD quality. I wet myself.