Microsoft have recently announced that they will now support Exchange 2007 SP1 running on a virtualised Hyper-V Windows 2008 platform.
There has been a new article published that gives you the information about best practices for running Exchange 2007 SP1 on Hyper-V.
If your thinking of giving this a go read this first has some very good information about planning for doing this in a production environment.
You must use Exchange 2007 SP1 on a Windows 2008 operating system to be supported
Note the Unified Messaging role will not be supported in a virtualised environment.
It's not just Hyper-V that will be supported which is a big surprise other vendors are being supported to see: http://windowsservercatalog.com/svvp.aspx?svvppage=svvp.htm
This is something that a lot of Exchange admins have been waiting to happen for a long time and a constant question i see banded around the Microsoft support forums so it's good to see Microsoft giving people the flexibility to use virtualisation with Exchange 2007 SP1.