Bungies weekly update included a hint that a playable version of Halo 3 might be shown at E3 next week. Words of Frank O’Connor from Bungie:

As for our next project, it continues apace. Lots of progress in every department, from UI to Animation to Engineering and of course, all the way back to audio. There are things to see in every department, some rough and ready and some astonishingly smooth, polished and even playable.

Halo 3 seems to be the only (cool) thing Microsoft can show at this years E3. Everyone focuses on Nintendo. A few on Sony.