Season 8

Season 8: Who's Staying, Who's Going?
POSTED 2-17-08
We've been getting a lot of e-mails lately saying "[fill in the name of the actor] has said they're leaving Smallville after Season Seven," which of course has caused some worry among the fans that their favorite characters might be on their way out, should the series be picked up for an eighth year.
However, our advice: Don't worry yet.
While it is true that Michael Rosenbaum (Lex), Kristin Kreuk (Lana), and Allison Mack (Chloe) are only contracted for seven seasons, it is always possible that they will negotiate for an eighth year. These negotiations could go on for a while, so it's possible we might not even know until late summer who's coming back - if there is even an eighth season (which, considering The CW's ratings in general, seems to be a no-brainer).
Tom Welling and John Glover are already contracted for Season 8; it is assumed that Erica Durance, Aaron Ashmore, and Laura Vandervoort are also contracted.
Until then... don't panic. While yes, it is possible that not everyone will be back next year if there is one, there are probably very few people that know exactly what will happen.
Source K-Site