Great news from Jeff on the Virtualization team blog, here is a short summary you can get the full details from here.
"The hypercall API enables partners to develop solutions with Windows Server virtualization allowing customers to achieve dynamic IT environments. These APIs are available for use by any organization seeking to integrate or extend their software with Windows Server 2008 and Windows Server virtualization."
"Microsoft is taking a step further in its commitment to interoperability by extending the Open Specification Promise to the hypercall API within Windows Server virtualization. With the OSP, any individual or organization is free to implement, commercialize and modify Microsoft’s virtualization format technology for free, now and forever..."