Games on Demand gets CoD4, BF2, Battlestations: Midway
Microsoft has added some war games to its Games on Demand service, in the form of the original Modern Warfare, Battlestations: Midway, and Battlefield 2.
You'll want to check for availability and pricing in your area, but in the US MW1 will run you $39.99, Battlestations $19.99, and Battlefield 2 will cost $19.99 as well.
You can get more information over on Major Nelson.