Hans Zimmer is a pretty prominent composer on the Hollywood circuit, winning awards for his music for such films as Gladiator, Pirates of the Caribbean and As Good As It Gets. He's even worked on a game before, contributing original material to The Simpsons Movie: The Game.
If the composer's listing (PDF link) at GSA Music is to be believed, he's also composing the score to surefire multi-billion seller, Modern Warfare 2. We've reached out to Infinity Ward for comment and will get back to you as soon as we can.
Source - Hans Zimmer's profile at GSA Music (PDF link)