If you've already registered via the direct link that some of our more rabid fans found earlier today, you may well have a perfectly valid registration, but from here on out you should only access the sign-up form through Halo3.com.
This is phase one. There will be more ways to enter the beta program, so don't panic. We will try to ensure that everyone who wants to participate has the opportunity to do so. The beta will be a world-wide affair, and every region that is elligible to participate, will. Much of the information on Halo3.com applies to the US only - so if you live in a different region, keep an eye on your local Xbox site for details.