Installing SBS 2011 as a guest inside a HyperV server parent is fully supported. Installing the HyperV role ON SBS 2011 is not. With the premium add on you can take the Windows Server 2008 R2 standard media and install it as a base for the HyperV role. Then you are licensed to install the SBS 2011 media AND the Windows Server 2008 R2 media as guests inside the Windows 2008 R2 server that you installed the HyperV role. Publish Post
This is referred to as 1+1 licensing and is a supported deployment of SBS 2011. Installing SBS 2011 as a guest inside any of the virtualisation platforms certified for Windows Server 2008 is fully supported. SBS11 on Hyper-V has an advantage in that there is no delineation of 'our problem' vs 'their problem' in regard to host/guest operation.
Installing the HyperV role ON SBS 2011 is possible but borders on insanity, and is also not supported.