When I was nursing Julien, he would go through phases where he would bite me(normally when he was cutting a tooth, and once it was through he would stop. As he got older (closer to 1yr he started to bite on purpose, so I would put him down and not nurse him) he quickly figured out that biting meant no food.
Kyle on the other hand, has started biting me much more frequently than Julien ever did, and at a much younger age. He isn't doing it on purpose, but if I take too long getting the boob ready for him, he will give me a little nip when he goes to latch on - almost as if to say I kept him waiting.
ahh, fun stuff.