From the SharePoint Team Blog:

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.

Follow up:

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.

We are working to release a hotfix to automatically fix this issue. A manual work-around is currently available and involves customers re-entering their Product ID number (PID) on the Convert License Type page in Central Administration. For more information and detailed steps please read this KB article.