It's a time-consuming process that can reportedly cost anything between $50,000 and $100,000 per minute of film but if just half the people who went to see Titanic first time around went to see it in 3D then it would still make an enormous profit for its co-distributors Paramount and 20th Century Fox. It's an opportunity that hasn't been lost on directors like George Lucas, who plans to convert all 6 Star Wars films to 3D, or Peter Jackson, who's keen to do the same for his Lord of the Rings trilogy.
Even films currently in production like Iron Man 2 are rumoured to be getting a 3D upgrade, such is the success of the format (though emphasis on the word 'rumoured'). AICN reports a 1 minute 3D demo of Iron Man 2 is being shopped around to 3D companies to see how much a full conversion might cost. There's also talk of the first Iron Man being converted and re-released in 3D ahead of the sequel but that seems unlikely given the short window of time (remember, Cameron said Titanic could take over a year to complete).
So far Toy Story 3D has been warmly received and with Titanic and Star Wars on the way there is certain to be an increasing flood of old movies put back on our cinema screens. Science-fiction and horror would be the obvious targets for 3D as they offer the best visual depth and opportunity for scares (Alien, Close Encounters, The Shining and Poltergeist would all be great in 3D for example). Frankly any reason for classics such as these to be seen on the big screen again is a good one, but seeing them in 3D will be a real treat.
Resources:
http://www.digitalspy.co.uk/movies/a167884/james-cameron-confirms-3d-titanic.html
http://gizmodo.com/5028874/confirmed-all-6-star-wars-movies-to-be-released-in-3d