It's going to be a fairly sweet system.
I reckon it is actually to pre-empt what Microsoft plan to do with XNA.. right now Steam is basically Xbox Live Marketplace for Windows; and what Games for Windows Live
should have been.
Be interesting to see how this all pans out.
Certainly will make developer life much easier, with a fairly simple "one-click" publishing and tool aquirement system.
I heard that Softimage|XSI will be released on it, but not much other news has been squeezed from Valve. They're totally going to screw Microsoft over on the PC market with basically the same product Microsoft are killing the other consoles with. Ironic and funny ^_^