Favorite smart remarks to responses to pregnancy announcement:

"Are you going to find out what it is?"

"No, we plan to change the diapers blindfolded and allow it to define its own gender."

"Was it planned?"

"Yeah, I had 'get pregnant' down in my daytimer right next to my dinner menus for the next nine months."

"Don't you know what causes that?"

"Oh yes, we know, but we're kind of wondering about you."
(Actually we haven't reached the stage where people ask this yet, but I heard that one from a usually demure mother of soon-to-be 9 who likes to use it on the purposefully childless.)