I new of a girl a while back who's front teeth were discoloured and she told me it was because her mum was on antibiotics when she was pregnant with her. Don't know if that's true or just a myth though
My niece also has a discoloured tooth and her dentist put it down to the same thing.

With the BF, I'd always been of the understanding that it wasn't an issue because when they suckle the nipple is drawn towards the back of their mouths and the milk doesn't really touch the teeth if that makes sense. If you are giving it in a bottle or a sippy cup then it is a different story.

HTH