This is interesting, if overdue: Microsoft's new beta release of the Windows Live Sign-In Assistant 6.5 includes the ability to link a Windows 7 user account to a Windows Live ID:

Windows Live ID Sign-in Assistant 6.5 (Beta)

Brief Description

The Windows Live ID Sign-in Assistant 6.5 (Beta) installs the Windows Live ID online provider for Windows 7. This provider enables linking a Windows Live ID to a Windows 7 user account.

Overview

The Windows Live ID Sign-in Assistant 6.5 (Beta) installs the Windows Live ID online provider for Windows® 7. By making it possible to link a Windows Live ID to a Windows 7 user account, this online provider helps enable Windows 7 applications to deliver great features like sharing documents, pictures, music, and video.

Benefits

Through the online provider, the Sign-in Assistant helps a Windows 7 application running on one computer identify and communicate with other computers that are associated with the same Windows Live ID. Just by linking your Windows Live ID to the user account on each Windows 7 computer you own, you enable those computers to identify each other and work together more easily than has been possible in the past. Applications can take advantage of this capability to support scenarios as in the following example:

On his homegroup, Bob wants to share certain documents with his wife but not with his kids. Simply by specifying his wife’s Windows Live ID user name, Bob can give his wife exclusive permission to access documents on his computer.

More Information
For more information about Windows Live ID, go to https://accountservices.passport.net/ppnetworkhome.srf?lc=1033.
For more information about Windows 7 Beta homegroups and sharing functionality, see http://www.microsoft.com/windows/windows-7/whats-new-possibilities.aspx.