Here are some of the 360's confirmed exclusives for 2008:
Gears of War 2
Fable 2
Banjo-Kazooie: Nuts & Bolts
Too Human
Viva Pinata: Trouble in Paradise
Tales of Vesperia
Infinite Undiscovery
The Last Remnant
Left 4 Dead
There's also Grand Theft Auto 4: Episode 1 which is slated for release between November 2008 - January 2009).
Halo Wars was originally expected to be released later this year but who knows when it'll be released (the same can be said of Alan Wake).