On some of my SharePoint 2007 farms, when I tried to activate the publishing feature, I got this error:
The trial period for this product has expired.
I was certain that my license had not expired, so I did some research on Google. I found that it was a known issue that affected Microsoft Office Servers product with service pack 2.
My SharePoint farms had been patched to version 18.104.22.16835 (SP2 + December 09 cumulative update), so they were affected.
The solution was to install Microsoft KB971620. It required a server reboot after installation.