As reported by Greg Kroah-Hartman today "hell has frozen over". Microsoft has released code under GPL (v2) for inclusion in the Linux Kernel.
It would seem that Greg through his work on the excellent Linux Driver Project has been working with some of the developers from Microsoft to make this happen. Described by Microsoft as "a break from the ordinary", they have released their Hyper-V driver (around 20,000 SLOC). They state that they have done this to "enhance interoperabilty between Windows and Linux" and "to provide the choices our customers are asking for".
I seriously hope that this is a signal of what is to come. As stated in Tux Radars article "A few years ago, Microsoft was describing the GPL as cancerous, so this seems like a major U-turn for the software behemoth".