Over on the SharePoint Team blog they have published details of an update that resolves the product expiration issue with SP2. A quick quote from their blog:

The update can be applied before or after Service Pack 2 installation.  If the update is applied prior to installing Service Pack 2 it will prevent the expiration date from being improperly activated during installation of Service Pack 2, if it is applied after Service Pack 2 it will remove the expiration date incorrectly set during installation of Service Pack 2.

Also they plan on releasing an updated SP2 package that doesn't exhibit this problem, time to update my slipstreamed install sources...