This is weird. I have one visitor to my site who gets the "Error: please type a comment" message every time he submits a comment. He's tried it in both Safari and Firefox on Macintosh, with the same result. No one else is having this problem, and the only way I can replicate it is to leave the comment box blank - which he's not doing.
Something seems to be blocking WordPress from recognizing there's any text in the comment box. Any ideas?