Microsoft has sent out emails to prospective testers of the next version of Windows Media Center XP. Neowin has a copy of the email text with a link to the beta site.
You sign into the site and are given a survey. It's about 40 questions about how you rate yourself as a computer user, how much tv, video, photos, and music you consume on PCs, and what kind of gear you have at home. Like any beta test, you never know who they are looking to help test, though it's usually to match some future market segment.
They say the test includes a PC, but doesn't mention if the new Media Center Extended hardware will be included, but it'd be silly to test just the OS and standard hardware without the new Extended features.