Listening to their fans, Turbine reversed an earlier decision, and is now making it possible for people from all over the world to play on US servers. You need to import a US version of the game to do so, but you won't be blocked from playing based on your non-US IP address. This makes it possible for international guilds to play together.
Well, nice of them. But I'll go with their recommendation, quote: "However, we strongly encourage players to purchase and play the game in their region to receive the best game experience, connectivity and local customer support." As previous MMORPG not always had European servers, I have experience with playing on US servers from here, and it isn't all that great. Your ping is much longer, making anything that requires fast reaction more difficult to do. And even if you are in a very nice international guild (like I was), that doesn't solve the problem that Europe is 6 to 8 timezones ahead of the US. You rarely really get to play with your guild mates, unless you play during strange hours. Raid starting at 3 am, anyone?