If you have been nursing all along, from what I have heard, it shouldn't change a thing. Your body is used to the hormones released from nursing and sort of ignores them as normal LOL For them, the only pregnancy I went full term without a lot of preterm contractions was the pregnancy I nursed all of the way through.
Not really. I nursed through all of mine (except the first, obviously) and had a wide range of birth-times, running from term (me being very well-nourished) to 5 weeks early (m being very poorly nourished).
Doctors, however, will tell you it can be a pre-term labor risk, and use that as a reason why you must wean immediately. I wouldn't wean, though.