Apparently there is a bug in Service Pack 2 of SharePoint 2007. 
During the installation of SP2, a product expiration date is improperly activated. This means SharePoint will expire as though it was a trial installation 180 days after SP2 is deployed. The activation of the expiration date will not affect the normal function of SharePoint up until the expiration date passes. Furthermore, product expiration 180 days after SP2 installation will not affect customer’s data, configuration or application code but will render SharePoint inaccessible for end-users.

See this page for more details and how to fix it.