As much as possible, I do not recommend virtualisation of SQL Server or any other RDBMS system, due to the impact on performance, and the availability of other alternatives such as multiple-instances. However following is documentation from RC0 of SQL Server 2008 Books Online disccussing the use of Hyper-V with SQL Server 2008:
SQL Server 2008 is supported in virtual machine environments running on the Hyper-V role in Windows Server 2008 Standard, Enterprise and Data Center editions. The virtual machine must run an operating system supported for the specific SQL Server 2008 edition listed later in this topic.
In addition to resources required by the parent partition, each virtual machine (child partition) must be provided with sufficient processor resources, memory, and disk resources for its SQL Server instance. Requirements are listed later in this topic.2
Within the Hyper-V role on Windows Server 2008, a maximum of 4 virtual processors can be allocated to virtual machines running Windows Server 2008 32-bit or 64-bit editions. A maximum of 2 virtual processors can be allocated to virtual machines running Windows Server 2003 32-bit editions. For virtual machines hosting other operating systems, a maximum of 1 virtual processor can be allocated to virtual machines.
· It is recommended that SQL Server be shut down before shutting down or terminating the virtual machine.
· Guest failover clustering (configuring failover clustering in SQL Server) is not supported in a Hyper-V environment.
· Failover clustering defined in the Hyper-V role in Windows Server 2008 (also referred to as Quick Migration) is supported, with SQL Server 2008 running in a virtual machine, when the configuration adheres to all the Hyper-V requirements.For more information about the Hyper-V role in Windows Server 2008, refer to http://www.microsoft.com/windowsserver2008.