from the looks of things the problem is that if u downloaded the client from bliz website for some reason the EU (EnGB) and US (EnUS)versions are available and u have to pick the right one. if you are using the eu version (EnGB) then you wont be able to log into the us servers yet because they have not been updated over there yet (scheduled for tomorrow). i dont know if there is a way to fix this yet or not but i guess redownloading the EnUS client may fix the problem. kinda bonehead move on bliz part tho, when you download the client from site it should automatically check your location and direct you to the right client to start with
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ENJOY GUYS!