I'm doubt Microsoft uses them extensively for internal applications, but i'm sure they have just about every OS installed in their corporate headquarters because they need to know what they are competing with.

Microsoft doesn't really have a solution for high-load DNS servers and routers, and Microsoft farms their DNS out to akamai so i'm sure you hit a few *nix servers on the way to accessing their websites.

Google uses Microsoft Office applications for their document management despite having Google Docs. That much I do know for sure

Microsoft does use Unix/Linux servers for a few of their production environments, but that is obvious to anyone in the field looking for a job. They recently had several positions open for *nix Sysadmins. I'll say this again: Microsoft makes great Desktop/Client software.

Its seems they still do not have a complete enterprise solution despite their size.

Yes. Hotmail ran on FreeBSD until the early/mid 2000s. Indeed, microsoft used to be a UNIX vendor back in the 1970s with XENIX. Also, most of thier internal DNS servers run UNIX or UNIX-Like systems.

Microsoft's DNS servers and distributed caching for Windows Updates and other downloads are outsourced to Akamai, a company that runs their operation on 48,000 linux servers around the world.

http://www.akamai.com/html/industry/software_technology.html