In an unexpected turn of events the additional map pack that was made available for the console versions of the game are due to be released on the PC on June 5th.
Although the map-pack was going to be available for the PC, no timeframe was given for it's release.
The additional 4 maps Broadcast, Chinatown, Killhouse and Creek where released back in April and could be purchased with 10 Xbox gamer points or you could purchase the Game of the year edition.
The final piece of good news is that the update including the maps will be free thanks to sponsorship by Nvidia (source).
Console to PC games
It's transparent where the market sector for this game is, sadly its seems games are ported to PC from console now-a-days. This results in some great console games appearing on the PC game market only to deliver poor results to the original FPS platform. COD4 on the PC is a good conversion even if the single player is a small element, but games like Frontlines: Fuel of wars appear to be badly thought out, for both single player and multiplayer.