What does this mean? Well, this means that Ledger's famous role will be taken up by someone else.
Moviehole's Clint Morris has claimed that he "heard from someone at the WB (Warner Brothers) that The Joker may be a part of the next Batman film - if even in a minor capacity. I believe it's definitely possible; ya never know. Heard some thing' were his exact words."
Now before you start thinking this is the worse decision that Warner Brothers can make, I need to stress the point that this is only a rumor and we can't get mad or happy that hasn't come out of the studio's mouth.
My opinion on this is that The Joker is a character I believe Ledger captured to the fullest. If they can find a way not to include him into the next film then do that and let the Ledger Joker live forever.
My other opinion is ... do we really need another Batman movie? (we know we'll get one, because The Dark Knight made to much money not to produce a sequel) I feel like that The Dark Knight gave us all we wanted in a Batman film and it's better if they just leave the franchise alone while they are on top.
I've also read many responses to this subject and I know the Joker is an iconic character in the Batman universe, and the most important villain, which brings me to the MTV interview with producer Charles Roven, who suggested that the role is more important then the actor playing it.
Roven is right, but I do believe if they do recast the role we know it would never reach the level of Ledger's and only be a disappointment to the fans, so I say let's move onto other villains.