Here's a very simple way to play Mafia II early on Steam for Non-US people, without the need to use proxies or other fancy third party programs:
Step 1: Pre-load the game by going into your browser of choice, and type in the url: steam://install/50130, let it finish downloading of course.
Step 2: Go to the ../Steam/steamapps/common/mafia ii/pc/ folder, create a txt file named steam_appid.txt, open the file, write 50130 in there and save it.
Step 3: Go back one folder (the mafia ii folder) and run launcher.exe.
There you go, Achievements work and it says on Steam that you are playing the game, the only downside is that you won't be able to use the pre-order DLC until the game is "officially" released on Steam for you.