I have installed lots of different Windows Servers and apps, including Exchange, inside VIrtual PC/Server, and VMware.
Rarely have I had any licensing trouble demoing/testing apps.
I’ll tell you, the easiest way to try Exchange 2007 is to download the VHD test drive directly from Microsoft. At that point, there is nothing to install, you just start it / run it, in Virtual PC. It is a limited time VM, I believe. Here is an article on that:
Here at TechTarget, there is a great article covering Exchange 2007 licensing.
I hope that helps.