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.
Check it out...http://sqlcat.com/whitepapers/archive/2008/10/03/running-sql-server-2008-in-a-hyper-v-environment-best-practices-and-performance-recommendations.aspx