Here is an email I received from ParentingWeekly.com, discussing teething and nursing. Emphasis is mine.
If your baby's teeth come in while you are still breastfeeding, you may be bit while nursing. This can happen for a number of reasons: if she's biting early in the feeding, she might be over-eager or frustrated, waiting for your milk to let down. If she bites late in the feeding, she may be sleepy and feeling her latch slip. Your baby's latch may also be affected if she has a cold or stuffy nose, and she may bite while adjusting. And, of course, your baby may just mistake your nipple for a teething ring and try to relieve her sore gums.
To stop your baby from biting while nursing, calmly and gently remove her from your breast and say "Don't bite Mommy" or "Don't hurt Mommy." Put her down gently or hand her to someone else, and wait a minute or two before you try to nurse again. This teaches her that when she bites, the milk goes away. Repeat this as often as necessary. However, always remain calm if bitten - never scream or yell. Startling or yelling at your baby may form a bad association with breastfeeding and she may refuse to feed altogether. Early and sudden weaning can be traumatic for you and your baby.
Early and sudden weaning can be traumatic?!? What, and I'm supposed to react 'calmly and gently' to having my nipple chomped on by several razor-sharp baby teeth??? Ha. And again I say, HAH! I've been bitten a handful of times now (usually when we're both on the verge of sleep... talk about a rude awakening!!!) and my response generally has been hollering, "OW OW OW!" which usually startles the baby long enough for her to break the latch and gaze disapprovingly up at me. Mother, she says, Really. I'm trying to eat here. Could you keep it down please?
Early weaning traumatic... hmph... grumble grumble...