Going by reports from Moviehole and Skewed and Reviewed, Arnold Schwarzenegger may indeed be back. According to producer Mario Kassar the script for the 4th "Terminator" film has been completed and they are waiting for Arnold to become available in order to put the film into production.
This is not the first time that Arnold has been attached to a future project; a third film in the "Conan" series has also been mentioned as an option. Schwarzenegger is currenlty the Governor of California and should he not seek/win re-election, speculation is that he will return to his action roots with "Terminator 4" being chief amongst his return projects.
Few details are known about the fourth film but SKNR learned that the film centers around the rise of the human rebellion under John Connors, and how they not only survived but eventually defeated Skynet. Though this could not be confirmed, it is likely based on the ending of "T3" that the future war will indeed play a factor in the next film.