The way I see it we've seen a great Joker story, and though the Joker could doubtless be featured again and again without it getting old, the main reason I would have imagined Nolan bringing him back for the 3rd installment would be to advance the plot of Batman and Gotham (i.e. 'letting slip' that Batman did not murder). If they can find a plausible way of advancing the plot in this way without him (if they're going to have batman redeemed at all), then I don't see any real need for a recast. Batman has loads of great villains and I disagree with the people who say the only way the climax can be exciting enough is to involve the Joker. Remember that this franchise will probably (and hopefully) go on long after Nolan has quit, and the Joker's not going to appear in every film, he's got to be put to the side at some point.
Also, I agree with Greg in that I think a recast of the Joker would completely take the audience out of the film, so unless Nolan deems it absolutely necessary, I'm against a recast.
Another point i wanted to bring up was with the people claiming that someone (Goyer?) said that there was a plan for the Joker to return in the 3rd part; wasn't that to scar Harvey in the courtroom? Seeing how he's already been scarred, wouldn't that make the argument that the Joker was meant to return null and void? (not that he definitely wasn't, just that he wasn't necessarily)
Having said all that, I'm not at all worried really because I reckon Nolan will make the best of the given situation, he hasn't gone wrong so far on many potentially hazardous decisions.