I actually just bought CoD4 on pc yesterday, and it arrived today. I had played the singleplayer and lan-multiplayer with a warezd version before, and I was so addicted to the multiplayer I decided to buy it. (That's somewhat impressive given the actual amount of games I buy instead of pirating).
Lo and behold when I searched for the latest patch online, I came across the remarkable CoD4MW-1.6-PatchSetup.exe which according to filefront contains the following:
"The Variety Map Pack includes four new multiplayer maps as well as the v1.6 patch which adds support for the new maps, fix a server crash exploit recently reported by the community, and a few additional features and bug fixes."
I'm downloading it at the moment, but my internet connection is too slow to confirm anything yet. I came across a couple of forum posts saying that it was sponsored by nvidia, and I think this is the official link.