Microsoft gets a licensing boost from Windows being run as a guest OS inside the virtual machine.  And interestingly, Mac OS X is run twice as often as Netware despite licensing and technical problems. 

The results indicate that Microsoft still stands to gain from the marketplace's rush to virtualization whether they have Hyper-V ready or not.

Guest OS usage by users:

96% Windows

52% Linux

30% UNIX machines

29% Solaris

12% Mac OS X

6% Netware

Information Week writes:

Virtual machines need an operating system with which to run, and the operating system most frequently being used in the current wave of virtualization is Windows, according to a survey sponsored by Sage Research and published in its recent Sage/CMB market Pulse newsletter.

Sage Research is the technology practice branch of custom market research firm Chadwick Martin Bailey. Sage surveyed Chadwick Martin Bailey's panel of technology service providers and panel of professional technology users and found 96% of the respondents use Windows on their virtual servers.

...

The findings were a broad brush stroke picture of server virtualization. They did not specify how many virtual machines of each kind was running on the virtualized servers or who the virtualization software supplier was.