From ComicsContinuum.com:

Cain, who starred in ABC's Lois & Clark: The New Adventures of Superman, is playing a character named Dr. Curtis Knox in the Smallville episode "Cure."

Here's how The CW describes the episode, which is scheduled for Oct. 18:

"Chloe (Allison Mack) learns that a doctor, Dr. Curtis Knox (Dean Cain), has been treating meteor-infected people and removing their abilities. Desperate to get rid of her powers, Chloe makes an appointment to see him, unaware that Dr. Knox has a nefarious plan to kill his patients and harvest their organs. Meanwhile, Kara (Laura Vandervoort) cons Jimmy (Aaron Ashmore) into helping her locate her crystal and defies Clark's (Tom Welling) order to stay on the farm after she discovers its whereabouts. Lana (Kristin Kreuk) sets up secret surveillance of Lex (Michael Rosenbaum)."