It looks like Valve is developing a game for the Xbox 360 according to this post
over on their developer blog. While they don't go into a lot of details about the game (I'm guessing we'll get that at E3 in two weeks)
they will be using a customized version of their Source engine to develop the new game. One would think that they might be bringing Half-Life 2
to the 360 but given that they ported it to the Xbox just last year I think we'll see something else.
Update: Gamers with Jobs contacted
Valve and it looks like Valve will NOT be announcing any details on this at the show this year.