I may have found the person who started "It's a girl!"
As some of you may remember, especially if you're Anglican/Episcopalian, the Episcopal Church in the US elected its first female Presiding Bishop, Katharine Jefferts Schori. She's also the first female primate (chief bishop of a province, not mammal of the order that includes monkeys and humans) in the Anglican Communion. Many wore pink buttons stating that "It's a girl!!" to her investiture. I believe I may have found the person responsible for the phrase:
"Update 23-Jun-06: Apparently my "It's a girl" idea got widespread use. There were even buttons at the convention. You can see one here at the elf-bunker (blogger is failing to upload images this morning)."