Is there guidance out there? Samples? Is this possible?
Everything I am finding is 2010 before WP7 was even released! (including this question: Windows Live Id Authentication for Windows Phone 7 and Windows Live ID Single-Sign-On on Windows Phone 7 and Getting the currently logged-in Windows Live Id (Windows Phone 7) and Messenger connect desktop API for Windows Phone 7)
Anything new?
yes you can, by using the windows azure access control service:
http://msdn.microsoft.com/en-us/gg192968