Erm. According to Joss, they haven't even nailed down the contract for Tony Head yet. And it's not going to be a series, it's going to be a BBC TV movie. But Joss does seem pretty optimistic that he'll get it done for a winter/spring premiere.
Also, while there's no official word on what the story is, most people agree that it won't be about the potentials.

Possibly a flashback story, with someone playing the young Ripper to get around the fact that Tony is a little old to play his younger self.
I can only hope they skip the BBC America route and send it directly to DVD for the US market. (Curses on you, Comcast, for putting BBC America only on digital cable so I can't get it!!!!)