From the Microsoft Interoperability Web site -

Microsoft Enhances Interoperability With Open Virtualization Format

BRUSSELS, Belgium — Oct. 17, 2006 — Microsoft Corp. today announced at the Interoperability Summit in Brussels that its virtualization format technology will now be available under its Open Specification Promise (OSP), an irrevocable promise from Microsoft to every individual and organization in the world to make use of this patented technology for free, now and forever when implementing specified open standards. Microsoft first announced the availability of an OSP for Web services specifications in September 2006 and now is expanding its customer-focused commitment to interoperability by applying the OSP to Microsoft’s Virtual Hard Disk (VHD) Image Format specification.

Source: Microsoft Enhances Interoperability With Open Virtualization Format: Irrevocable promise will make Microsoft’s virtualization format software freely available to all.