Well I am sitting here waiting for my notebok to install the latest VISTA SP1 BETA I was surfing the web and ran accross a good article on integrating CardSpace into you ASP.NET applications, check it out.
What is CardSpace? Well a gross simplification is its X.509 certificates mushed with vCards, or another way to think of it is (Kim Cameron would probably say this is wrong but whats the chance he will catch me saying this 
) as a framework, tools and set of experiences that (normally) enable self signed certificates (not x509) as a form of user authentication.
One of the things I really like about CardSpace is that they have tried very hard to hide the dirty details in a attempt to make them usable; I actualy think they would make a great authentication technique for 10 foot applications like Media Center Add-Ins.