LoginRadius looks very interesting but I have two concerns/questions about it:
1) How does it handle/integrate registration of new users in a site where a username is needed? Ideally it would give the user the option of using their login username (such as their Facebook username or whatever login they are using) OR a new username for the site on which they are registering.
2) I think users can find it confusing with these types of logins whether the logins are actually registering them on the site or just doing a one-time login.