The answer to this one is not immediately available on the web, so I thought I would bring together the conclusive evidence that I did find online and answer this question once and for all. It was a recent topic of discussion in the TOPDOG04 household after a repeat of the Nicodemus episode aired. Turns out I was wrong. It was a body double. Kudos to Ms. Kreuk for standing her ground. (Kristin Kreuk plays Lana Lang on Smallville for the WB).

FAN SERVICE ALERT!Whoa, now that was one gratuitous episode of "Smallville." In a nutshell, a weird plant's pollen made people lose their inhibitions, act crazy, and then (if their name didn't appear in the opening credits) take a dirt nap. It was essentially a reset of the old "Star Trek" episode "This Side of Paradise."
The writers made up for this lapse in creativity by parading Lana Lang (played by Kristin Kreuk) around in fuck-me boots and a short leather skirt. Oh yeah, they also had her (or more likely her body double) strip down to her skimpy red lace lingerie, dive into a swimming pool, and later make out with a solidly befuddled Clark Kent.
[Note: I'm not saying it couldn't have been Kreuk herself showing off a remarkably toned figure, but there were strategic cuts obviously intended to avoid showing the person's face.]
Here is an excerpt from an AOL sponsored chat with Kristin from May 2002, reposted at Comics Continuum:
On the pool scene in the "Nicodemus" episode: "That was a body double in the pool scene. I read that script and I freaked out, because I didn't want to objectify myself like that."They obviously didn't change the scene in the writing, so I said, fine, just get a body double and I'll work with the rest of it.
"I was definitely not comfortable getting into a red lace bra and underwear for that scene, and I wasn't, so I didn't. I'm really happy with that decision."
An article in the September issue of Vancouver Magazine backs up the chat transcript.

And if you're interested in more about Kristin and her views on women being objectified in the entertainment industry, you should read this interview about a photo shoot she did for Arena Magazine.
What else can I say? Kristin, will you marry me?