Just seen the advert, truly awesome. Everyone @ work's talking bout that game! (Santa, if you're reading, I want a XB360 - nice one).
The answer is an easy one, if you've seen (and freakin' love!! like me!!) the film Donnie Darko.
Originally released by Tears For Fears, the version in the advert for Gears Of War is by Gary Jules, and was number 1 at Xmas last year, I think. It's called "Mad World" - personally, I prefer the Tears For Fears version.