Re: 9 month old biter!
I don't think biting back teaches anything except that people DO bite!!! Babies don't have the cognitive development to think, "Wow, if mommy bites me and that hurts, it probably hurts when I bite other people!"
My suggestion is to firmly tell the baby "No" and set him down away from whoever he bit for a minute. He'll get the picture that if he wants to interact with others, he better not bite or he'll be taken away every time!
Wife to Terry (7.21.01), Mom to Grace (6.18.03) and A.J. (10.10.05)