I've seen wp-snippets.com uses Twitter sign-in which somehow creates a WordPress users with the Twitter credentials.
wp-snippets.com > signin with twitter > Oauth > CallbackURL = wp-admin with twitter credentials written to WP user
How is this achieved? I've searched a lot plugins and cannot see anything that writes twitter user details to the Wordpress database. I expect if any that the Otto's Simple Twitter Connect (STC) might have, but I'vve installed and examined it; I cannot see this feature anyway.