If you have installed the latest Vista build you may have noticed the "Offers" section on the Welcome screen.
Some good pimpage there for the Windows Live services which again keeps them high profile ;)
Anyway - One of the offers is for the "Windows Marketplace" which takes you to a website (http://www.windowsmarketplace.com/) which is a new Windows property where you can buy software online. Windows Marketplace features a piece of software called the Digital Locker which like Steam is positioning itself as a major new channel for electronic software distribution on the web.
So using the Digital Locker you can try and buy software online and from a single location even though you are likely sourcing from a number of vendors. If you change PC - just revoke your old license and then reapply it to the new PC. So it manages that for you as well.
Currently they have a number of games up there from Direct2Drive like Civ4 or WoW.
Anyway, all that aside, this is actually the launch for a project which was undertaken by one of my fellow RD's - Scott Stanfield from Vertigo. Great stuff Scott! :) Apprently he is putting up an article on MSDN shortly talking about how they made it so check it out.