When you log in to the game for the first time, you will have the opportunity to pick any available server, or home world. If you choose to select a home world that is in a different region from where you purchased your game, you will be logged out for a short period of time while your account is moved to your new region. Once you have selected a home world, all the characters you create will play on this world and will represent it in our world-versus-world (WvW) format.
So the game is bought in on either the US or EU territories (depending on where you live). One is able to choose from the server "worlds" either under US or EU territories; nice..
When Guild Wars 2 launches, you will also have the option to play with your friends on another world with our free “guesting” feature. With guesting, your characters can play on any world where you have friends—with certain restrictions. For instance, you will not be able to participate in WvW while guesting.
How long does the guesting last?
Will one be able to log-out and log back into the same world while being guested?
If guesting works as seamlessly as district hopping in GW1, I don't see PvE being a problem whatsoever.
I am going to assume that no matter where a member is, everyone still contributes to building guild.
In general though--even though small--I don't like the idea of splitting community(guilds) between servers unless they pay. Splitting guilds up this way--especially for WvWvW oriented guilds--will just keep the guild faction-ed off rather than feeling like a whole.