I'm inclined to say much or all of the background is indeed all rear-projected. If you look at the top-right around 5:06 or 5:07, you see what looks a lot like it might be light-emitting text floating in midair. (EDIT: Correction! It's not just floating there, it scrolls left just as the camera is coming down, I missed that on the first viewing. So it's definitely not just on a physical banner.)
As for "massive resolution", slicing up a framebuffer and shooting out the components to multiple projectors wouldn't be a new idea, and I'll bet that's what was done here.
Yes, it was all projected. (also the side walls of the theatre). Some 20+ projectors pushing 300 million pixels/sec. The intro to the keynotes was pretty amazing as well, meshing the projection with light effects and live performance: http://www.youtube.com/watch?v=VrDPgUjqTQ8&feature=relat...
How does it disappear at the end - or is that a virtual digital overlay?
Wait, is the entire background rear projected, like a borderless movie theater screen? Must be massive resolution ?!