Season 6 episode 6/16 entitled "Promise"
Early, I know. But I believe this will be a big episode, and we should begin speculating now.
According to K-Site, this WILL be the Lex and Lana wedding episode. Also from K-Site:
Spoiler:
UPDATED 2/2/07: Someone we have not seen on Smallville in a few years will be attending the Lex and Lana wedding and guest starring in the episode. Start guessing now...