I heard the same thing, listed on superherohype.com. After following a few links, I found the exact text (since I was wondering how Kevin Smith knew, since he's not involved with the movie, though he is big into comic books)..

Quote
"I heard Singer's thinking about [Kevin] Spacey as Lex Luthor, which is awesome," Smith told The Post.
Guess we'll just have to wait and see. wink

"You need me. You wouldn't be much of a hero without a villain. And you do love being the hero, don't you. The cheering children, the swooning women, you love it so much, it's made you my most reliable accomplice." -- Lex Luthor to Superman, Question Authority, Justice League Unlimited