According to an interview the Avatar director did with the New York Times, James Cameron says he encouraged George Lucas to go ahead with the upcoming Star Wars 3-D conversions. Here are a couple of the quotes provided by Digital Spy:
"George has been talking about this for a long time and it's finally coming to fruition. I've been encouraging him to do that for years. He did hang back for a while. I think he was waiting for there to be a wide enough install base in movie theatres, and now we're looking at the roll-out of 3D television."
"I think he's timed it wisely to a point where he can do the theatrical re-release and then roll right into a 3D DVD re-release. You don't want to create a glut of your own product. He had only just, a couple of years ago, finished his whole epic cycle of six films."