We told the kids that we were going to have a new baby just before Christmas and Emma and Nathan are really excited. When we told them Emma started jumping up and down and was saying "YAY!" Nathan said, "ok, I'm going to go play now," and off he went. Since then he has showed that he really is excited about the new baby.
Than when my mom came over Sunday I had Emma tell her our news. I whispered to Emma, "tell Grandma about our surprise." Emma just looked at me like I have no idea what you are talking about. So I whispered again, "Remeber the surprise daddy told you the other day?" She instantly lit up and said, "We have a new baby... in Mommy's tummy!" My mom looked totally lost for a minute and then it sunk in. Although she was definitely shocked, she seemed happy for us and excited to have another new baby in the family.
I would have to say shock is definitely the most consistent reaction, but people do seem happy for us.