Just to clarify, this isn't from the trailers. An interview from MTV Multiplayer reveals it:
"He also mentioned to me that the game will be scored primarily with hip-hop. His favorites are Kanye West and Jay-Z, but he didn’t say they’d be part of the official soundtrack."
http://multiplayerblog.mtv.com/2008/05/23/madworld/
Kanye is one of my favorite rappers, and there's a lot worse than Jay-Z, so I'm pretty happy with this decision.