Character creation and realms

[Centaur]

What is confirmed, is that your characters will be in a global pool (so not characters per server), that there will be possibilities to switch servers, and that by default you will login to the same server that you were on at the end of your previous session. The rest is based on small tidbits of information that were given in various interviews with regard to what they're aiming at. And there is no official stance on this yet, as these details aren't finalized yet.
As for switching servers for WvWvW... yes, people will definitely be able to switch servers, however, ArenaNet plan to have some sort of restrictions in place to prevent server hopping to always get to the server with the bi-weekly bonus. Whether that means a limit on how often you can switch, a time limit during which you don't receive the server bonus, or even some kind of negative bonus after switching (so that you don't benefit from that server's positive bonus)... or something else, has not been announced yet.

To quote Martin Kerstein:

The plan is to make switching servers as painless as possible, so they can play with their friends while not unbalacing the entire World vs. World PvP system. We are still working on the details.

[Mintana]

I'm not certain, but im pretty sure that you can use characters on different realms, not just one like on WoW.