SQL Server 2008 on Windows Server 2008 Hyper-V Performance Guidance
Wow this is great day. The SQL team has worked really hard in creating a performance guidance document with solid suggestions and performance numbers when running on WS08 Hyper-V.
They looked at OLAP and OLTP workloads. Even if you are not running SQL it will give you and idea of how other database like workloads are likely to perform.