The Xbox.com site doesn't promote them very well. For those that don't know, you can get the following from Xbox Live:
Gears of War 2 "Bare Your Teeth" Theme
Gears of War 2 Crimson Picture Pack
Also available on Xbox Live are the Gears of War 2 "Duel" Teaser Trailer, and GDC: Gears of War 2 Storms the Stage featuring Cliffy B, Lead Designer at Epic Games, where he states: "Gears 1 was a set up. It was an appetizer for all the characters, the environments, the game play mechanics we introduced in the first. Gears 2's going to be bigger, better, and far more bad-ass then the first game in every way."
You can also find the videos at the Xbox.com Gears of War 2 page