This is one of the coolest things ever.
But let me tell you why:
So far most of the Live services have been pretty cool. Office Live is pretty neat, but I'd like to see support for Visual Studio. FrontPage is cool and all, but why no love for VS? But back on topic, Single logon is awesome. I really wish more developers would dig into AD and use this feature. Now this single sign on will be extended to include more stuff that I use everyday like Live Messenger and Office Live. I'm sure there'll probably be some sort of way for third party devs to use this just as there is for the current Passport system. Hopefully, it won't be as friggin difficult to use. The Passport system that MS currently uses grabbed my interest until I started looking into actually implementing it. What a PITA.
RE: Microsoft Plans Link Between Directory, Live Services
Microsoft plans to sync its Active Directory with its Windows Live Web-based services to give users a single sign-on for applications and services both inside a company network and on the Web.
[Via ActiveWin.com Headlines]