If you look closely at the headers, you'll see that www.microsoft.com is being handled by a server reporting as “Microsoft-IIS/7.5”. IIS 7.5 is the Web server in Windows Server 2008 R2, that also includes IIS Extensions such as FTP, WebDAV and the IIS Administration Pack. Windows Server 2008 R2 also provides full ASP.NET support on Server Core installations, includes a new Web Administration module and IIS cmdlets for Windows PowerShell 2.0.
David Lowe reports from the Windows Server team blog, "Many of the HP servers on which Microsoft.com is hosted require no updates or reconfiguration, and we expect that customers with supported Windows Server 2008 hardware should find the same. ...
"We are already running a sizeable chunk of Microsoft.com on the newer version of Hyper-V that ships in Windows Server 2008 R2."
For more information, see Microsoft.com Now Running Windows Server 2008 R2 Beta on the Windows Server team blog. You can try out Windows Sever 2008 R2 beta for yourself.