Update: The fix has been released!
SharePoint Server Service Pack 2 activates the Trial mode of all SharePoint Server related products (SharePoint Server 2007, Search Server, Project Server, etc.). The workaround is to simply re-enter the license code. If you do not do this, then the product will deactivate after 180 days of installing SP2.
However, if you’re running Microsoft Search Server 2008 Express, no license code is provided.
Microsoft will be releasing the June Cumulative Update for SharePoint Server which will fix this issue. The update is expected to be released on June 30th or the first week of July. It is available under KB971620.