J.P.
LOL - I just went through a similar experience with my 2 1/2 year old and me being 4 months pregnant. She started "self-weaning" when I was about 10 weeks and now she's completely off. She said "milks gone." My milk started slowing down dramatically when I was about 7 weeks pregnant so it didn't surprise me. She didn't seem to mind at all. Also, I knew I was probably going to wean her this summer before I even knew I was pregnant so I started explaining to her that "momma's milk" was for babies and she's not a baby anymore.
The other day she was watching me get dressed and she asked me "what are those" and I replied " you know what those are." She smiled and said "momma's milk for babies" and I said "yes, will you help mommy give your new brother/sister momma's milk" and she said "yes." I'm sure it won't be as easy as that when the time comes but I figure its best to prepare her as much as possible. Plus, for the most part I think we don't give them as much credit for knowing what is going on as we should. I think they know and I think just as adults, sometimes they choose to ignore it =)