Apparently this news originated at Variety during an interview about a possible Inception video game(as reported on in Ed's article below) but there seems to be no trace of it now. I picked up the story at Bleeding Cool who report..
Yes, he’s writing another Batman, which is still to be officially greenlit (of course). Nolan even made a joke on the matter:
They haven’t told me they don’t want to do it, so I’m hopeful.
I wouldn't take this as a sign of ill omen for another movie. Its not unusual for a studio to hold of on this type of official info. Look at
The Avengers! Marvel still haven't issued any kind of press release confirming Joss Whedon as director, he had to reveal it himself at Comic Con.
Anyway, aside from the film's expected release date of July 20th 2012, we have had no other official announcements regarding Nolan's third Batman flick. Nolan has also said that Mr Freeze definitely won't be the villain! Thanks for that Chris, I'm sure you eased many a fan's mind there. Short of that there have been rumors of Joseph Gordon Levitt in talks to play The Riddler, and talk of either Catwoman or Talia Al Ghul being female villain/love interest.