From io9:

First and foremost Nolan “emphatically and unhesitatingly” stated that The Joker will not be recast for the next film, “I just don’t feel comfortable talking about it.” He also stated that Mr. Freeze [which he’s stated before] and The Penguin are out of the running for Batman 3’s villains.

Considering Ledger’s truly excellent portrayal of The Joker, I’m thrilled that Nolan believes recasting the character would be a mistake.

Secondly, two of the stupidest and comical characters in the Batman universe— Mr. Freeze and The Penguin— are completely out of the running for the third film.

Let me be the first to say… thank God for that.