My kiddo is just over four months old and has his first real tooth coming in (he started teething at 4 weeks, but it came and went for a while, nothing serious), and it is SHARP! And he bites me... usually when he's frustrated or worse, when he's bored, at the breast. I have just been reacting by saying NO (in a startled, scary voice, which I don't mean to do, but it always scares the heck out of me and hurts like heck too, so it's just sort of reactionary) and putting my breast away for a bit, or putting him down for a bit.
Any other ways you experienced mamas have dealt with this? He seems to think it's hilarious and laughs at me when I do it, which is not helping things on either end here.