Is there any proof that fluoride in drinking supply is bad for your teeth?

No: Fluoride decreases cavities by 60%. Overfluoridation causes extra white spots and occur in children by eating toothpase not generally from fluoridated water(use only a pea-sized amount and only at age that they can spit well - 3 or over). Healthy teeth are crucial to overall general long-term health. Fluoride does not cause cancer.

Overdose only: Dr orton's answer is excellent. Since the mass introduction of Fluoride in the 1950's, the percentage of children recieving Fluoride and having no cavities has skyrocketed, compared to children not exposed.And all data reports no cancer issues despite skeptics.