I was thumbing through some of the sites listed on : http://www.dotnetopenauth.net/case-studies/
Although the dotnetopenauth website claims that the sites use dotnetopenauth, I found many of them used a traditional login/registration process and I could find no evidence of an openid Relying party signin process.
I assume, at one time, these sites were actually using dotnetopenauth, and for one reason or other switch away from it.
StackOverflow is the first site on the list, and I know that I can still login to SO using my OpenID credentials. Is SO using DotNetOpenAuth, or are you using some other code base?