My 2.5 year old still nurses 2-3 times in a 24 hr period. He goes through the night without it, which was a major struggle, but he still likes to nurse to sleep. I can see that being a problem when the baby is born, although my sister still nurses both her toddler and her 7 month old to sleep at night. Not at the same time, though.
With my oldest (who self-weaned at a little over 4) the nurse-to-sleep session was one of the last he gave up, even though he had been going to sleep by himself at naptime for a long, long time before that. Sometimes I feel guilty that I'm depriving ds2 of the chance to wean on his own schedule by getting pregnant. I know he would not have night weaned yet if I hadn't insisted. But then I've always regretted not being able to give ds1 a sibling closer in age, so I guess if it's not one thing it's another.