The VHD format specification is now available for download. The specification contains all the technical details for reading/writing and modifying VHD images. This has a lot of potential, and can be used for things like backup, antivirus scans, image management, disk conversion, and others. The spec was released under Microsoft’s Open Specification Promise:

As of Tuesday, October 17th 2006, Microsoft is providing access to the VHD Image Format Specification Document as a part of the Open Specification Promise (OSP). The OSP provides broad use of Microsoft patented technology necessary to implement a list of covered specifications. The goal of the OSP is to provide our customers and partners with additional options for implementing interoperable solutions. Please reference the OSP Website for complete details.

Link to the Press Release: Microsoft Enhances Interoperability With Open Virtualization Format
Link to download page: Virtual Hard Disk Image Format Specification