If you are planning to run Exchange 2007 on a virtual environment, think twice before following that path:http://technet.microsoft.com/en-us/library/bb232170.aspx
From the previous article, these are the messages that you have to consider:
As an additional note, in future Hypervisor (add-in to WS2008) will support 64-bit guests systems, but still we’ll have to wait for a confirmation from the product team if it’s going to be supported on virtual environments.
For those who are implementing HMC 4.0, the same rules apply and moreover MPS is not supported in a virtual environment. That doesn't mean that the solution won't work, because it does, but later on, if you face any issue (i.e. SMTP queues growing, services hanging, etc.) MS support won't be able to warratee a solution, unless the issue can be replicated in a physical server.
Hope these considerations help you to take the right decision while deploying Exchange in a virtual environment