Yep. According to one of the Mac mags (I forget which one, useful or what?) it's a bug that strikes at random. From what I can remember of the article, there's no fix yet. Other than being irritating, it doesn't seem to affect anything.
The other problem some people are having with the Mail programme is losing the mac.com account sometimes. Twice I've had to turn it on again from the e-mail tab in the internet control pane. I believe that was in in the same article. If I find it, I'll post.