Most of you folks might have seen this already but it is never too late to mention an important fact.
Microsoft opens the Virtual Hard Disk Specification under its own Open Specification Promise (OSP) with the aim of encouraging development and increasing interoperability. In case you want to learn more about Microsoft and Interoperability in general, please visit the Microsoft Interoperability homepage. OSP is part of Microsoft Interoperability.
The Windows Server Division blog has more details about the VHD spec. Or you can go directly to the download website.