According to Buddy TV, William Petersen gave an interview to British Radio Times about him leaving CSI. While it seems vague it does seem impending. He also seems very critical of the whole franchise. The one thing that seems to hold him back is his responsibility to all who have a job through what he does. Maybe he realizes that without his character there is no CSI. If he leaves I wonder if they will try to replace his character and possibly let the show die a slow death or if they are just going to close up shop.
(HT: Chris from SHS group)