Windows Home Server SDK has just been released
http://msdn2.microsoft.com/en-us/library/aa306570.aspx
Interesting, Microsoft has released a beta SDK for Windows Home Server. I would have thought that this product would be a fairly closed system, but there is the documentation for extending it. The other interesting thing is that the docs discuss building apps in C#. I would have thought that any extensions would have been in the form of unmanaged C++.