Microsoft have been beefing up their own internal Halo team, 343 Industries, as of late and the most recent addition is former Kojima Productions composer Sotaro Tojima. Tojima has worked on Metal Gear Solid 4: Guns of the Patriots and Castlevania: Circle of the Moon while at Konami.
Does this mean the next Halo game after Halo: Reach won't be using Martin O'Donnell's tunes? Perhaps... we'll just have to wait and see.