buy.guildwars2.com is now live, allowing you to pre-purchase your copy of the game and get access to a few bonus features, including a 3 day headstart and access to every beta weekend event.
It also allows you to register you account, which syncs up to the original GW and access the new forums.. But there seems to be heavy traffic trying to actually register at the moment.
Personally, I'm a little surprised they're letting people pay upfront for a game with no release date. Although I have a lot of faith in the developers, that's something I see putting a lot of people off. So for those uncomfortable with this, you can still just pre-order the game the old fashioned way, which should still get you a 1-day headstart (although you'll lose the guaranteed BWE access)