Michael Rosenbaum had said that this would be his last season on Smallville and he has kept his word and won't be returning as Lex Luthor next season. Though a statement from the CW hints that we may see him again in some fashion, which would be great. In his place they are going to bring in Doomsday and an unnamed female villain. Well, hopefully we will see more of his great character as Lex in guest appearances and I'll be looking for him in his new endeavors.
Here is the letter to the fans about his decision to leave:
Dear Smallville Fans,