Infinity Ward's Community Manger (Robert Bowling) has posted some good news on his blog about the forthcoming CoD4 Variety Map Pack. Nvidia is sponsoring the June 5th release which means that the map pack will be available to PC users for free! Hurray!
The map pack will include four new maps which are named Broadcast, Chinatown, Killhouse, and Creek. There will also be a patch (1.6) to allow the new maps to work while also fixing a server crash issue that has been reported by the community.
The map pack has been available for the consoles since April, so if you PC users want to see what these new maps look and play like, check out the videos in this IGN article.