I tivo'd it last week and just finally watched it today, I love stuff like this and I'm pushing 40.
Anyways, yeah it has an element to it that may seem salacious, but it's not over the top like Gossip Girl(for a kid that is). Most of what goes on in the show is implied not shown(the actions listed by the other poster).
My 13 yr old likes stuff like Life Unexpected and Vampire Diaries and they definitely have their moments, but those moments have led to some really good eye-opening converstations between the two of us.
When I was 12 I was pretty darn smart regarding sex, drugs etc, I knew what was what. I had the reading comprehension of a freshman in college and frequently sought out books that were mature in nature. But I also loved shows/books that were geared to my age. Twelve yr olds aren't stupid that's for sure, they know what's up and out there and a lot of it may not even disturb your daughter. Especially if she's a rather mature 12 yr old.
She might just find herself intrigued in the mystery the whole show is centered on and not pay too much attention to the rest. And from what I've read on some boards regarding the series itself, I don't think the pot stuff and parent cheating stuff comes up anymore. Except maybe in discussion as having ocurred.
Watch it with her tonight and see what you think. I honestly have little issues with teen shows in regard to sex, I'm more inclined to censor with violence, since sex to me isn't that huge of a deal.