Actor William Peterson is leaving the acclaimed TV show ‘CSI’ after eight years starring as Gil Grissom on the CBS hit. The decision seems mutual by both parties but not extremely permanent. He will leave sometime towards the middle of the upcoming season.

“Billy is leaving,” confirms the show’s executive producer, Carol Mendelsohn. “But he will remain throughout the run of the series an executive producer. And he will, whenever CBS asks, come back. I don’t think you’ve seen the last of Gil Grissom.”

Neither does Petersen. In fact, in an exclusive interview conducted on July 11, the eight-year CSI vet seemed almost reluctant to characterize his departure as, um, a departure. Citing both his ongoing role as an executive producer and his intention to return on occasion as a (very special) guest star, he said, “I’m in a great place in terms of knowing that I’ll be more free to make choices. And I’m responsible enough to not do it in a way that would hurt [the show].

The show will survive but will face a challenge in maintaining its ratings dominance with Peterson’s depature coupled with those of Jorja Fox and Gary Dourdan.

CBS has to be concerned about that, too. Although no one expects ratings to nosedive when Petersen leaves, the network’s top-rated show is staring down its biggest challenge ever. His sorta swan song, coupled with Jorja Fox and Gary Dourdan’s exits last season, brings to three the number of original cast members CSI will have lost in a year’s time.

Peterson has made so much money off of this franchise that it is hard to believe that he will stay away from it too long. He probably wants to devote his time to other projects or perhaps even a movie. He could retire now and live the high life off of the ‘CSI’ residuals for the rest of his life. I say he is back sometime in 2010 on a more regular basis.

Actress Lauren Lee Smith, from The L Word, has been added to the upcoming season of ‘CSI’.

UPDATE: Laurence Fishburne Possibly Joining ‘CSI’ Cast