Since the posting, Major Nelson's blog has instantly been flooded and gone down, you can try accessing his site from here, or you could click here for a direct link to the sign up page. From there you just put in some basic info and wait in hope. Apparently there is plenty of room in this preview, so if you sign up in the next few days, it looks like you're guaranteed to get in.
This update looks pretty cool, I imagine some features, such as YouTube and TV services won't actually be available in the preview since they probably won't be ready. What will be available for sure is the new interface. While it might be not much an improvement for people with controllers, as a Kinect user, I'm quite excited by the idea of being able to do pretty much everything without a controller. Give it a sign up, you might get in.