Keith Mars writes a book about the experience of cracking the Lilly Kane case with the reporter to whom he gave all the evidence in the season finale. He's going on a book tour, and they're currently in discussions with Larry King for him to appear (Keith would go on Larry King Live as part of the tour).
According to Rob Thomas, there's a scene in the premiere where Weevil confronts Veronica about dating an oh-niner and says something like, "How was your year of living dangerously?" There's going to be more of the class struggle this year, including a specific event in the season premiere that exacerbates the conflict.
Also, there will definitely be another season long mystery.
Charisma Carpeter is slated to be in at least seven episodes. Steve Guttenberg will be in at least six. As for the DVD, it's coming out Oct. 11 and will have the original pilot that critics saw (which apparently liked better) and deleted scenes.