Finally a whitepaper released regarding virtualising SQL Server.  

This white paper describes a series of test configurations Microsoft ran, which represented a variety of possible scenarios involving SQL Server running in Hyper-V. The paper discusses our results and observations, and it also presents our recommendations. Our test results showed that SQL Server 2008 on Hyper-V provides stable performance and scalability. We believe Windows Server 2008 Hyper-V is a solid platform for SQL Server 2008 for the appropriate workload. It is practical to run production workloads under a Hyper-V environment, as long as the workload is within the capacity of your Hyper-V guest virtual machine.

Link to whitepaper: Running SQL Server 2008 in a Hyper-V Environment - Best Practices and Performance Recommendations.