^^She has her own show on the Disney Channel, "Hannah Montana"; she's also the daughter of Billy Ray Cyrus. I'm guessing her teeth look like that cause they're still actually her own teeth, and she hasn't succumbed to Hollywood veneers...
*ETA: She's also only 14 y.o., so I don't think she's anorexic looking, as someone else suggested, she's just a growing kid.