Just so you folks are aware, there's a guesting feature (which will be active after the period of free server transfer ability at launch ends) that allows you to visit and play with friends on other servers. So, beyond guilds and WvWvW, you'll be able to play the game with anyone, regardless of their server.
You also won't be able to create characters on multiple servers, I don't believe. Your account is tied to a single server as your home server. But, again, it largely won't matter due to what I mentioned above.