OK, here's a possible reason. The plugin is working on my other blog at Quirkysex.com and I didn't upgrade that one to 2.1.2. It's still 2.0.3. So perhaps the newest version of WP doesn't support this meta tag plugin.
There are some extra plugins I'm using on Ms Naughty: Sociable and Chicklet creator. I deactivated both and it made no difference. So it would appear to be the upgrade that's done it.

I'm now going to do a bit of searching on the WP help site to see if I can find more info.
Edit: Well, there ya go. It's not listed here:
http://codex.wordpress.org/Plugins/P...lity/2.1#L_-_O
I've found four other plugins that should add meta tags, but I don't know what it means for all my previous posts and their painstakingly-added descriptions. They might be useless now.