Hans Zimmer. Check the composer of big Hollywood films, and you're bound to find his name. He's scored movies as diverse as the smash hit The Lion King soundtrack and his Badlands inspired True Romance score.
Most recently, Zimmer has scored Frost/Nixon and The Dark Knight.
According to Zimmer's agency, the Gorfaine/Schwartz Agency, the composer is writing music for Call of Duty: Modern Warfare 2. The agency lists the game on Zimmer's resume.
Nothing has officially been announced yet by the game's publisher. We are following this up.
Update: According to The Hollywood Reporter, it's official.