Well, damn. Guess I helped start a war last week.
Somebody posted to a web design mailing list I read, asking "whether anybody still used .html these days", as opposed to the .htm extension.
Hell yes, I thought, and wrote a post about it. The gist of it was: what extension you use doesn't really matter very much, but I'd recommend using .html, unless you specifically know your client will want it on Windows 3.1-compatible disks, or something. The reason? Well, first that it's technically correct (those files with .htm extension are still written in the HTML format), and second that - to me - .htm gives a bad impression. Seeing the .htm extension is a little like seeing "geocities" or "aol" in the URL - kind of a sign that somebody maybe doesn't know what they're doing and chose the default, easy, Microsoft-sponsored option. Of course, that's only a very brief initial impression and is going to be far outweighed by the actual content of the page.
Unfortunately, everybody who read the message managed to read: "USE .HTML OR DIE."
I think about five people posted to say that it doesn't really matter (which I'd already said). Another five posted to say that they used .htm, and that didn't mean their websites were rubbish (which I hadn't claimed). And there were two or three messages with actual useful information, about an old CD-ROM standard which only supports 8.3 and is apparently necessary to ensure Mac/Win/Unix compatibility of CDs.
I suppose it may have been my own fault for using the phrase "LOSER ALERT" in my post. :) Even so...