Ridley Scott (Alien, Gi Jane, Gladiator) - In my opinion he is probably the best of the choices. He has a built in following among fans of sci-fi and epic movies in general. He can do action without loosing character development. He has wanted to work with Schwarzenegger for quite some time. But he is quite busy, and being talked about as the director of a new Jerry Bruckheimer movie.
Christian Duguay (The Art of War) - He is the least experienced of the choices. He has mostly done TV movies before The Art of War and may not be up to the task of following in James Cameron's foot-steps. Working in his favor is the fact that he would be the cheapest of the choices. He could also be picked if producers Kassar & Vajna want someone that they can push around, and won't have to fight for control.
Roland Emmerich
David Fincher (Alien 3, Seven) - He is the most artistic of the choices. His work devides people into those who really love it and those who really hate it. Working against him is the fact that he has directed 4 movies and had only one big hit (Seven).
John McTiernan (Predator, Die Hard, The Hunt For Red October) - Another good choice. He has had several hits, and knows how to handle action. He has also worked with Schwarzenegger two times before (Predator and the under-rated Last Action Hero)
Jonathan Mostow(Breakdown, U-571) - He has only directed 2 big movies, but both of them were able to keep audances on the edge of their seats.
I'm pulling for Ridley Scott myself, however Duguaay has the least commitments
According to the current rumors the story will be about the nuclear holocaust taking place. As August 29, 1997 (Judgement Day) has already happened perhaps the story will say that the distruction of Dyson's Lab, and his death, caused it to take longer to develop the technology than it would have if he had been on the project the whole time.
When they made T2:3D (1996) Cameron said that the story would not be just a knock off of the Terminator series. He wanted to use it as the jumping off point to a third Terminator film. After some consideration he opted out of this movie, and it is unknown if the people who are doing T3 will follow T2:3D's continuity or just act as if it never happened.
T3 page at upcommingmovies.com
The Internet Movie DataBase page for T3
The IMDB is a great source for info on movies that have been released but it is not a good place for info on upcomming movies. The IMDB runs on user tips and is too big to fact check every rummor that someone sends them. So far they have listed 4 diferent directors as THE director of T3. It is a good place to find links to Terminator fan sites though.
A Fanfic sequel to T2
WARNING, WARNING DISCLAIMER UP AHEAD: This is a fanfic sequel. It was written by a guy who likes the terminator movies. This is not a link to the real T3 script. I am including this link only because it is a good script by a good amateur writer. Who hopes to someday become a professional writer.