A couple of weeks ago VMware published a benchmark analysis titled Performance and Scalability of Microsoft SQL Server on VMware vSphere 4.
It assesses the performance of a SQL Server 2008 OLTP database hosted by a vSphere 4.0 virtual machine with 8 vCPUs and 58GB vRAM, and compares it against the performance of a physical system:
Of course it would be much nicer to see how a OLAP database performs, to understand the reliability of virtualization in extreme (and real-world) conditions, but the chances to have it seems pretty low.
Nonetheless the paper above is extremely interesting and really worth a read.