For what its worth @JNapodano has an estimate of peak sales of 1.5B and DCF fairv value model of 23.25. I don't follow VVUS but I tend to like Jason's reports (the few I've seen anyway).
I think Matt Herper's arguments that the stock is fairly priced include patients don't stay on diet drugs long and no obesity drug has had 1B of sales (I don't know if that is accurate or not).
EDIT: Here is a link to his model (hope he doesn't mind me posting but its on twitter so anyone can see it) http://twitpic.com/8o2er9/full