No, not likely at all. If Nolan saying previously that he wouldn't include Ledger (or Joker) in the third Batman film out of respect to the actor and his legacy, then Warner Bros. also wants you to know that it ain't happening. A source for the studio told Moviehole that the Ledger rumor is "just not real", and while a much earlier, pre-'Dark Knight' storyline involved Joker returning in the third film to help Batman catch Two-Face, Ledger's death immediately put an end to that.
And speaking of ending things, Nolan also confirmed to EW this week that 'The Dark Knight Rises' would indeed be "the last chapter of our Batman saga." Obviously that won't be the end of Batman, with rumors of a TV series kicking around and Warner Bros. possibly resurrecting the 'Batman vs. Superman' idea. Once Nolan is out of the picture, though, expect Warner Bros. to feel out the situation and bring back the Joker as a live-action character in future Batman productions as soon as they feel the time is right.