The VDS API is now live on MSDN!   

What is it? VDS offers a COM-based API for volume, disk and LUN management in Windows Server 2003. In Windows Server 2003 R2, we also have additional integration with iSCSI targets and MPIO. For example, you can use it to add a drive letter to a LUN.

P.S. The VDS SDK is not yet on http://downloads.microsoft.com but you can download it from http://www.microsoftstoragepartners.com after you complete an additional registration process.