I’ve started watching my web server logs more closely, and found several requests for /_vti_bin/owssvr.dll and /MSOffice/cltreq.asp. Examples:
Apparently, these requests are caused by someone using IE with the Discussion Bar turned on. I wonder why IE isn’t smart enough to read the headers to see that I’m not running a Microsoft web server. Or maybe MS has fixed this with version 7 of their browser, since all of the requests I see are IE6.