This month's BBM has a pretty good article on martial artists who teach the stars, but in usual BBM fashion they only touched the tip of the iceberg (I only bought a copy cuz I needed something to read while having some tests at the hospital).
They talked about Pat Johnson, Tak Kubota, Richard Norton, Jerry Poteet, and Benny Urquidez. The story more or less underlined what I always felt to be the case, that the stars were taught specific techniques in specific scenarios to serve the needs of the picture (at least in movies with actual movie stars, and not "martial arts stars").
There are a GREAT NUMBER of martial artists who work behind the scenes making stars look competent as martial artists on film, especially in the roles of body-doubles and adversaries. Micheal DePasquale Jr. even offers a training school for doing just that!
In his day, Ed Parker was one of the most prolific advisors on MA action the industry ever had. Without him, those WONDERFUL rumbles between Clouseau and Kato wouldn't have been near the fun they are!