http://www.apb.com/Click the bubble to register interest. You'll get an email telling you to add a bunch of emails to your contact list, to insure you receive an invite. I have no idea if this will be a PC beta or 360 beta. Probably a 360 beta if it isn't a beta for both.
I expect APB to be a insanely badass game. The faster this game is released, the faster Crackdown 2 can get worked on.
