Yeah its disappointing to have paid for a game and only being able to play half of it. Its not looking good for being able to spend my weekend playing Uncharted 3 multiplayer.
I'm having the same problem as reported. I'm a UK gamer, using a UK game on a UK machine, logged in to a UK account. I used the code on the back of the manual, bought the game new and sealed from Asda, and tried entering the code so many times I could tell you it by heart.