If you are getting the error “Trial Period for this product has expired “, whether slipstreamed or updated, verify ALL of your IIS Application Pool Process accounts are in the local administrators group - not just the Server Farm Account. Otherwise, you will get a DCOM error (in eventvwr.exe) when connecting to the web application. RTM isn't supposed to require this, I will update this post when I can verify. I really don't want all of my process accounts with local administrator privileges, so I hope this is fixed.
HTH,
-ben
UPDATE
The latest versions of MOSS should not require local admin. Just add all of your application pool identities in dcomcnfg.exe, to IISREGWAM with Launch and Activate.
--Update 1-9-2008--
Joel Oleson has blogged the latest info on this error:
http://blogs.msdn.com/joelo/archive/2008/01/02/evaluation-version-expired-but-not-really.aspx
Ben Curry, CISSP, SharePoint Server MVP
Mindsharp
http://mindsharpblogs.com/ben
http://www.microsoft.com/mspress/books/10623.aspx