Hi,
This has been widely reported, but I've not seen any mention of it here so 
in case people don't know:
   
http://press.redhat.com/2009/07/21/red-hat-welcomes-microsofts-kernel-contribution/
Microsoft has offered a significant contribution to the Linux kernel under 
the GNU General Public License version 2. This is important news. It seems 
like only yesterday that Microsoft was declaring Linux, open source 
software and the GPL to be the axis of evil. Now Microsoft is making a 
credible opening bid to become a member of the Linux community. As the 
largest corporate contributor to the Linux kernel, Red Hat would like to 
acknowledge this and encourage Microsoft to continue on this path.
The new contribution consists of three Linux device drivers directed at 
interoperability. They are designed to allow Linux to run as a virtual 
machine on top of Hyper-V, Microsoft's hypervisor. Such interoperability 
is to Microsoft's benefit, of course, as a selling point for Windows. But 
it's also good for Linux users, in providing more choice and flexibility. 
So it has win-win elements.
There is more at the URL above, but that's the summary, and while Red Hat 
seems to welcome the contribution others are certainly more sceptical of 
MS's motives.
   Andy