Sorry to resurrect an old thread, but I followed the instructions here and it worked beautifully across my site for twitter hovercards.
That said, people have a tendency, in the forums and shoutbox, to specify @something when they want to refer to a user. This leads to hovercards that aren't actually for the right people being displayed.
Is there a way to restrict the hovercard so that it only works on the profile icons? Not the entire site?