Even with screenwriter David Goyer debunking all rumors about who might be cast as the supervillain in the next Batman film, that doesn't stop new casting rumors from coming out. This time around, the word has it that Rhona Mitra (Doomsday) is up for the role of Catwoman in the film, at least according to Batman-On-Film.
Now, they're quick to point out that there is still no "official" script for the film. But fans of Batman are no doubt doing a run-down in their head about the most famous and interesting of the Caped Crusader's villains, and realizing that odds aren't bad that Catwoman will be in the next film. So, it makes sense that rumors as to who might play her are coming up.
Furthermore, it's entirely possible that either the studio or Ms. Mitra's own people have started the rumor, simply to gauge fan-interest, and to see whether or not their would be an audience for the film based on her casting. It's no secret that part of the financial success of The Dark Knight came from people who wanted to see Heath Ledger's final performance as the Joker. So, it makes sense that studios might "test the waters" to see what sort of buzz would be generated by a particular casting rumor.