"I'm calling it: Libby's a plant. She's the Ethan of the Tailies' camp.
First off, go to the tape. Libby's the last one to interact with Cindy. And Cindy's carrying a large blue knapsack. By the time they've climbed to the top, Libby's carrying the knapsack. You're telling me she took the knapsack from Cindy and then proceeded to forget all about her? This is a ridge, not K-2 — you don't just ''lose'' people like that. Also notable: Cindy hands Libby a walking stick of some sort, just before we lose track of her completely. As this handoff takes place, we see a cavelike opening in the background, right where Cindy's about to pass. And it looks as if there's some sort of shape inside.
But here's my best evidence of Libby's treachery: Tonight, she told Sawyer she's a clinical psychologist. Well, who appears to have designed this tropical house of horrors, according to a certain orientation video? A team of clinical psychologists, that's who. Coincidence? What do you think? Is there such a thing as coincidence in the world of television drama? Consult the little Locke on your left shoulder (as opposed to the tiny Jack on your right)."
There is also speculation that Claire is the new plant in the Jack/Locke group.