I entered my OpenID URL, but after I've logged into OpenID (at my provider's site) and approved of StackOverflow, I was redirected to StackOverflow and it used this as the OpenID URL:
<my entered URL>/index.html
I don't think this is correct, since the moment I change my site to have a frontpage other than 'index.html' I can't log into StackOverflow anymore.
To clarify: the site - because of the webserver - shows the content of index.html, since there was no page specified. But that's no reason for stackoverflow to use '.../index.html' as OpenID URL. I know some other sites (and API's) that don't have this behavior and I think it's not really desirable to have this.
It might also be interesting to know that OpenID definition on the site is a delegate that refers to my real provider URL, so maybe it has something to do with that.