The low-hanging fruit of server virtualization is consolidation of relatively low utilization servers. Does this mean a high utilization server, such as an enterprise Exchange implementation, should not be virtualized? Exchange can be virtualized, but IT should know the virtualization business goals going in.
If the primary goal is workload consolidation, then a high volume Exchange server is probably not a good candidate for virtualization. However, if the goal is to bring the availability, management flexibility, and recovery benefits of virtualization to the exchange implementation then Exchange can be a legitimate candidate.