Starting today MSDN and TechNet subscribers will be able to download Windows Home Server 2011 for testing and evaluation purposes.
Make sure you also check out the Windows Server Solution SDK
site, which overviews how to create add-ins that can extend the functionality of Windows Home Server.  Also just released is the Windows Server Solutions SDK Installer
, a tool that helps you install the Windows Server Solutions SDK. It has Visual Studio 2010 samples and templates that you can use to easily create applications for Windows Home Server 2011.
Stay tuned for further information on the evaluation experience for Windows Home Server 2011 releasing later this month.