Beginning this Thursday, April 8, gamers around the world can play Call of Duty: Modern Warfare 2 multiplayer free of charge during a special Free Weekend promotion on Steam. The definitive online experience, Modern Warfare 2 returns with a host of new perks and enhancements. Those who wish to enjoy every moment of the Modern Warfare 2 multiplayer Free Weekend may pre-load Modern Warfare 2 multiplayer now. There is no obligation to participate.
Not really, Steam have done it with plenty of games (started with TF2 I think) and i'd guess it works fairly well.
Might be to try and boost sales for the DLC as I'm guessing that comes to PC soon.