That’s why they are attempting to close a deal with Michael Rosenbaum to return as Lex Luthor for at least 9-10 episodes!

Lex Luthor is finally returning to Smallville
The Superman Homepage received word that Brian Doherty, who was just this month promoted to svp, national sales, at The CW has revealed that the execs at The CW are excited about a 10th season for “Smallville”, wanting to really “go out with a bang”.
Update on Smallville's renewalThe word is that they’re attempting to close a deal with Michael Rosenbaum to return as Lex Luthor, wanting to sign him up for at least 9 or 10 episodes.
Ausiello Renewal scorecard - "Smallville" is a sure thingApparently an announcement will be made one way or the other very soon.
Source: Superman Homepage

is great idea
This would be awesome. Zod and Tess are great but the show is still missing something. if they brought back Lana for part of the 10th season that would really be great. She was such a huge part of Clark’s life she should see him finally become Superman. Remember in an early season she said “what are you man or Superman?”
Noooooo. The show was okay with Lex, but got sooooooooooo much better once his storyline was done. Why else do you think ratings are up?
I think this is a good thing. We can see more rivalry. And we can also be reunited with a familiar face. Just as long as he has a good plot. Lex cannot be dead. The writers and producers can’t sway from the original Superman mythos that much, right? Whatever they decide, I’ll be waiting for season 10 and I’m gonna love every second of it.
I’m totally cool with them bringing Lex back. As long as its not Lana! I do miss Lex, Michael is such a wonderful actor its a pity not to see him anymore.
Ok LISA, first of all the ratings are up because Clark is finally becoming Superman like he is supposed to, it has nothing to do with the fact that Lex is gone. Lex was the best part of the show, the only person that needed to go away was Lana. If they bring back Lex, they will have the highest ratings in the 10 year history of the show
if michael could just say yes and resume his role as lex luthor ,i hav no doubt that the ratings would go through the roof.michael’s portrayal of lex luthor far exceeds that from any other superman based series/movie.not lana,i got tired of her during the early seasons it always felt like she was holding him back.her presence for 10 is only going to do the same.
I never really thought Lex was dead, you don’t get to where Lex is without learning how to side-step a few land mines. I will be glad to see him return, I have been a loyale fan since season 1, and am very upset that this will be the final season. There are so many possibleitys for Clark to meet the future Justices League. He could meet up with Batman, or Greenlantern. The possibilities are endless.