I'm not Catholic, but it seems odd to be to send out an announcement for adults. If I got something like that in the mail, I would wonder if I was obliged to send a gift, and that would make me feel awkward.
DH and I were baptized in our church last year, and we just told close family and friends. I probably posted in on Facebook as my status of the day as well. But we didn't send anything out.
I would be more inclined to just send out invitations, even for family who are too far away to attend. I would also put something like "Best wishes only, please" on it so that nobody felt obligated to send a gift. Putting it in a Christmas letter after the fact, like a pp suggested, would also be a good choice.