This won't really apply to any of you, but just make sure you're getting the new Call of Duty 4 maps from a PSN account that matches your region.
Don't use your phony UK account to buy them for your US game.
Here's the scoop from fourzerotwo:
"If you have the GERMAN version of the game, you MUST purchase the GERMAN version of the map pack.
If you have the UK version of the game, you MUST purchase the UK version of the map pack.
Just make sure that the Disc version and the Map Pack version matches up. If you have an UK version of the game, DO NOT purchase a German version of the map pack. It won’t work. The same goes for any other mismatched version."
What do you do if you've already purchased a mismatched version?
"For those of you who may have already purchased the incorrect language for the game disc you have, we are currently talking with Sony regarding any options you may have."
We don't think this will apply to any of you, but good luck if you messed up.