We wouldn't display the at-sign. But, hey, we don't display the HTML anyway... people would just have to learn it over time.
As to how to do other things: well, that's pretty easy. @URL is going to be an OpenID account (if it exists). @user.site is going to be a user at another site. @news.livejournal.com (local OpenID account) vs @news.lj (remote user).
no subject
As to how to do other things: well, that's pretty easy. @URL is going to be an OpenID account (if it exists). @user.site is going to be a user at another site. @news.livejournal.com (local OpenID account) vs @news.lj (remote user).