In case you have not seen this already, a hot fix has been released to fix the Service Pack 2 issue of setting your license key to a trial one. Here is the original post from Stefan.
Microsoft has released a fix for the SP2 problem that reverts the license into a Trial version after installing Service Pack 2.
More details can be found in the following KB article:
971620 - When you install the 2007 Microsoft Office servers Service Pack 2, the product expiration date is activated incorrectly
As the link to the fix binaries in the KB is currently broken here are the correct download locations: