Nolan had this to say over at ign.com
"We're not addressing the Joker. That is something I felt very strongly about in terms of my relationship with Heath [Ledger] and the experience I went through with him on The Dark Knight. I didn't want to in any way try and account for a real-life tragedy. That seemed inappropriate to me. We just have a new set of characters and a continuation of Bruce Wayne's story. Not involving the Joker."
Granted, Ras wasn't mentioned in The Dark Knight, I just find this a bit disappointing. It's always unfortunate when the number one route most directors take in respecting actors who have passed away is to never mention their characters at all. I'm certainly not asking for a recast, but the Joker was a pretty huge deal.