From the Microsoft press release:
REDMOND, Wash., July 20, 2009 - Today, in a break from the ordinary, Microsoft released 20,000 lines of device driver code to the Linux community. The code, which includes three Linux device drivers, has been submitted to the Linux kernel community for inclusion in the Linux tree. The drivers will be available to the Linux community and customers alike, and will enhance the performance of the Linux operating system when virtualized on Windows Server 2008 Hyper-V or Windows Server 2008 R2 Hyper-V.
Microsoft wants Hyper-V to compete with VMWare in all markets, and to do this it needs to have good support for virtualizing Linux. Microsoft very pragmatically decided that closed source paravirtualization drivers for Linux had no chance of success. They'd get a press release out of such a move, but no significant adoption without Red Hat/Canonical/etc pulling the drivers in. Opening the source is in their best interests.
The last two posts have been an experiment of sorts. Prior posts had been written entirely from scratch on a technical topic. They take a long, long time to write. I wanted to try posting more frequently by adding a few thoughts to a relevant news item, but thus far I haven't been happy with the results. In this article I said opening the source would allow a Linux platform vendor to include the Microsoft paravirtualization drivers, but on further reflection it seems unlikely that they would actually do so. Red Hat has their own virtualization strategy which doesn't include Microsoft, and there is no reason to believe Canonical would be interested in being an enabler for sales of Microsoft Hyper-V.