According to this, Activision will release new multiplayer maps for Call of Duty 4 this spring for both the Xbox 360 and PS3 versions. There are no firm details on what to expect from the update.
Furthermore, if you're American, you can get the chance to go to Infinity Ward to see the new downloadable content being made. All you have to do is... download CoD4 to your mobile. "No purchase nec," says the ad, just above a "buy" button next to the game. See here.