The public update to fix this issue is available for download.

Post URL is here: http://blogs.msdn.com/sharepoint/archive/2009/06/25/service-pack-2-updat…

Service Pack 2 Update.
The public update for the Service Pack 2 expiration date issue is now available for download.
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.
The update is applicable to all of the products that this issue affected (see the list in the KB linked below).
Installation instructions and download links for x86 and x64 are available in this KB: http://support.microsoft.com/kb/971620
The direct download link for x86 is: http://download.microsoft.com/download/2/F/5/2F51AB71-1325-49D2-9CB9-18D…
And for x64: http://download.microsoft.com/download/5/B/B/5BBD34A9-C528-42B0-8A5F-9A8…
We will be updating the existing Service Pack 2 download package with a new package that includes this fix within the next 4-6 weeks.
Details prior to this release..
What can I do to fix it?
To work around this issue customers will need to re-enter their Product ID numbers (PID) on the Convert License Type page in Central Administration. Please see this KB article for detailed steps. (The KB link is not currently active, it will be available within the next 48hrs)

Premier customers may find additional details at http://support.microsoft.com to resolve this issue.

Product ID numbers (PIDs) can be retrieved by logging into the Volume Licensing Service Center.

Note: For Search Server 2008 Express the only way to resolve this issue is to apply the hotfix.

Do I need to update all of the servers in my farm?
No. The Product ID number only needs to be entered once.

What if I haven’t installed SP2 yet but want to? What should I do?
You can continue to plan for and install Service Pack 2 for SharePoint Server 2007 with the additional step of re-entering your Product ID number after the installation is complete by following the instructions in this KB article. (The KB link is not currently active, it will be available within the next 48hrs)

Does this issue impact anything else?
The issue only activates the product expiration date. The activation of the expiration date does not affect the normal function of SharePoint up until the expiration date passes, at which point SharePoint will be inaccessible for end-users. This issue does not affect customer’s data, configuration or application code.

I can’t find/don’t know my original PID, where can I find it?
Product ID numbers (PIDs) can be retrieved by logging into the Volume Licensing Service Center.