I just noticed that you can download the Alpha (interestingly they use that instead of the usual “Community Tech Preview”) release of the Windows Live Id Client 1.0 SDK. This comes with documentation and a simple application that demonstrates authenticating using your Live credentials. Scott’s got a great article on getting started with the SDK.
Across on the forum there have already been questions raised about whether this can be used to authentication for websites and on mobile devices. My burning question is whether they have planned to support occasionally connected applications? What happens when my connection fails – does it log off immediately? do I need to log in again? Can I logon when I’m offline (assuming I’ve previous logged on using this computer in the same way as the operating system)?