Windows SBS 2008 comes in two versions, regular and Premium, with one of the key differentiators being that Premium includes a second Windows Server 2008 Standard license, intended for a SQL Server install, but suitable for other uses as well.
Does that second Windows Server 2008 Standard instance include the right to install one additional virtual Windows Server 2008 Standard inside Hyper-V, just like a retail Windows Server 2008 Standard includes? THat would mean that one possible scenario could be as follows:
One physical server, running Windows Server 2008 Standard (the bundled license) with the Hyper-V server role installed, and SBS 2008 virtualized, as well as a second virtualized Windows Server 2008 Standard to run SQL Server.
This could be a simple way for a company to plan for growth and business continuity by allowing either instance (or both) to slide off one server to another as the load increases or if hardware fails. Of course, this would require buying a single server that can ahndle not only the current load of both virtualized servers, but also the overhead of virtualization.