There are rumours that it's a well-known Irish actor. And for some reason, Chris O'Dowd has been strongly rumoured recently. I'm not sure whether that's a case of O'Dowd is particularly rumoured, or just a case of, "who's a well-known Irish actor? Oh, I know..."
Honestly? Much as I enjoyed The IT Crowd, I can't really see him as Doctor material. Still, keep an open mind, and all of that.
However, just recently, someone posted this picture of Robert Sheehan (formerly of Misfits, although he's done a load of other stuff which I'll admit to not having seen).
He's Irish. He's a good actor - from what I've heard, a great actor. Okay, he'd be the youngest Doctor by some way. But, in a "channelling his inner McGann/Pertwee" he definitely looks the part in this picture...
We'll find out on Sunday. If it doesn't leak before then (and...I wouldn't be surprised if it does. This has not been the BBC's year for Keeping Things Secret...).