my little one (6 months) got me GOOD yesterday! she didn't feel like nursing, apparently, and clamped down as if to make a point. her tooth just, just cut the day before, and it's sharp as hell. she is much more feisty than her older sister so I'm really hoping she doesn't think she can use that as a tool!!
my toddler only ever bit once (as a baby) and I don't even think it was on purpose - she had finished nursing and I wasn't paying attention.