From the european forums of World of Warcraft, about the possibility to play on the Euro server with an american account and vice versa:

“At this time, owners of the US copy of the game are unable to play on European servers.”

“You will have to buy an EU version to play on EU servers. Theres no way to transfer US accounts to a EU server.”

“Once we have an official policy on this, it will be posted on all of our websites, American and European. At this time, the process of transferring players from American servers to European servers, or vice versa, is under review. Mostly this is due to the imbalance of character levels between server blocks. It wouldn’t be fair to allow a level 60 character to be transferred to servers where a majority of the population is still level 20 or below, and it wouldn’t be fair to allow some players to transfer, but not others – thus we are reviewing the best single solution to give to all players who wish to transfer.”

This doesn’t make sense. The players are asking just about the possibility to access the servers and build new characters when, instead, the answer is about moving the characters (the players?) directly. Imho it smells of pay-per-use service. It also sounds like you won’t be able to play on both the euro and the american servers, you’ll have to make a choice and move your whole account.

We’ll see, it won’t happen before a few years considering Blizzard’s pace of development.