Re-latch. A very common reason for biting while nursing is the baby is not latched on correctly and much more obvious when baby get teeth. If a baby is properly latched, with the nipple far inside his mouth and vigorously nursing he can not bite you. It is when they stop and your nipple slides that they bite. Try to anticipate it, if he stops nursing, gently remove your nipple and have him re-latch.
Remove from breast. If an infant is biting while breast feeding usually removing the infant from the breast and uttering a sharp "no" from the mother is all that is needed to stop this behavior. Babies are developmentally incapable of understanding that their behavior is painful but they can learn to stop doing it.