I think the biggies will be:

GTA4 DLC unveil.

Banjo Kazooie 3

Alan Wake

GOW 2

Cry On

Maybe a Forza 3 trailer( Or let us know it's in Development)

And I have a hunch that the Remote will be announced and playable with about 5 - 10 games. The likes of 360 sports, Maybe a Kart game from Rare(As Diddy Kong racing was good). Banjo Kazooie Racing. They want some Casual market to gain ground over Sony. As MS said they have something major for the 2nd half of the year.

all of the display units will rrod :(

Secret Rare game? Can those developpers seriously afford to split their resources?

Microsoft did quite a bit at GDC, so I think we can expect more of an expansion on that, with maybe 1 or 2 announcements about new 2009 games.

A new IP announced. Kameo 2, Perfect Dark 2, Banjo Kazooie 3, Halo Wars and Gears of War 2

Yeah I forgot about Cry On. It would make sense to hype their next big MS published JRPG. Forza 3 is a genuine possibility, though I doubt it would be out this year.

Kameo 2 and Perfect Dark 2 would be EXCELLENT!!!

^Trolling? How does that contribute to a mature discussion with a statement like that?

I predict Microsoft will drop out of the console race...

Nevermind, that was a dream I had.

Bungie will annouce marathon (I hope so at least :P)

Halo 4, because they need it.