This is true ... at the same time, though, if the user has anonymous commenting enabled, there's nothing to stop him or her from just dropping links in the body text. (Or is there?)

Hey ... what if the website field had some kinda AJAX-y thing where it auto-detected if the URL was to an OpenID-enabled site after you'd typed it in, and then let you know immediately afterwards if it'd work? This would especially help for if anonymous commenting was disabled; you'd type in your website, and if that didn't work you'd read the error message and maybe scan over a list of sites that would work, before deciding on one to enter.