SharePoint error when activating publishing feature: the trial period for this product has expired

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 12.0.0.6535 (SP2 + December 09 cumulative update), so they were affected.

The solution was to install Microsoft KB971620. It required a server reboot after installation.