Alot of circumstantial evidence is now pointing toward the Zodiac being a completely different suspect than any of the ones showcased in this flick. Considering the source material was based on Graysmith's book; which is pretty close-minded in it's approach to nailing down a suspect, I think it's kickass that Fincher exercised such a level of open-mindedness about his approach to the material.
Graysmith is utterly convinced Allen did the crimes, and there's alot of factual errors in his books that he either overlooked or purposely fudged to help him butter up his POI. For Fincher to adapt that source material and still make a movie that doesn't conclude in any kind of definite way means that he probably did alot of outside research to get himself cozy with the case. Absolute kudos to that guy; the movie would have made another 30 million bucks had the last act been Graysmith chasing Allen through an abandoned power plant.
Fincher took the high road with the case and stuck to the facts; paid for it at the box office, but I'm glad to see more and more people are finding this movie as time goes by.