This is one of the toughest issues a newly pregnant mom can face. I have a friend that faced this about a year ago. Her toddler nursed/co-slept with them. What she did was slowly get him to sleep in his own bed. She started with the crib in their room. As he got used to sleeping in it all night, and not nursing all night, they moved the crib into his own room.
When I found out I was pregnant with #2, I gave dd#1 a pacifier, she was 9 months old. She took to it pretty easy and stopped waking for me in the night. I continued to nurse her until I was 20 weeks pregnant and my milk left.
This age is tough because there is a lot of seperation anxiety associated with it. Make sure whatever you decide to do, you go slow, and easy. Your baby is extremely attached to you. Of course, your duty is to the new baby.
If you are interested, La Leche League has a ton of articles on their website,
http://www.lalecheleague.org , and I've found several on weaning, on pregnancy and breastfeeding, on sleeping issues. I'm sure you can find lots of info to help you make your decision.