It looks like the trinity of Lucas, Spielberg and Ford did not all like Frank Darabont's screenplay for the 4th Indiana Jones film.
Instead Jeff Nathanson has been tapped to give the screenplay a shot. Nathanson is the writer of such gems as Rush Hour 2 and Speed 2, but to be fair, his more recent efforts have been much better as evidenced by The Terminal and Catch Me If You Can.
I'm curious what it was that wasn't agreeable about Frank Darabont's script. He's penned a number of episodes of the Young Indiana Jones TV series and such excellent films as The Green Mile and The Shawshank Redemption.
I guess the main problem I see with these continuing delays is that by the time the movie finally makes it to the big screen, Harrison Ford will be in his late 60's. Also, if they stick to their earlier premise of real time transpiring between the last movie and the next, it will take place in the mid-1960's.
This is complete and pure speculation on my part, but I wouldn't be surprised to see a younger actor appear in the film, to whom Ford could pass on the old whip and hat to continue the franchise. Seems like something similar to that is going on in the upcoming Blade: Trinity, where they're bringing in a couple of twenty-somethings as Wesley Snipes' proteges.