http://blogs.technet.com/scottschnoll/archive/2006/12/31/exchange-2007-platforms-and-product-keys.aspx
"Evaluations and Product Keys
When you install Exchange 2007, it is unlicensed and referred to as a Trial Edition. Unlicensed (Trial Edition) servers appear as the Standard Edition, and they are not eligible for support from Microsoft Product Support Services. The Trial Edition expires 120 days after the date of installation. When you start the Exchange Management Console, if you have any unlicensed Exchange 2007 servers in your organization, Exchange will display a list of all unlicensed Exchange 2007 servers and the number of days that are remaining until the trial edition expires. If you have expired unlicensed Exchange 2007 servers you will also see a separate warning for each expired server. For lab, demo and test environments, unless you have a valid reason for rebuilding the environment, or unless you just love our new Setup wizard so much that you just can't stop uninstalling and installing server roles, I recommend that you get used to dealing with the expiration nag dialog, and not rebuild your servers every 120 days. Either way, the choice is yours, but again, you won't lose any functionality when running on an expired Trial Edition. "
