Instead of just sending out cards, which is what everyone does, why not think about getting some personalised cards made up with personalised chocolates on them? Everyone loves a miniature chocolate, it's a nice gesture which isn't corny but still adds a touch of fun and personality to your baby announcement cards.
You could consider getting them made up with your new baby's name and birth date, and maybe his or her birthweight as this seems to be the common thing done... alternatively you could use the personalised chocolates to carry a message from one of your older children (if you have any) saying something to the effect of, "James is proud to introduce his new baby sister Lucy," and most personalised chocolate services are able to print photos as well as messages on their wrappers, making it something that will be able to be unwrapped and stuck up on fridges or soforth. If you were holding a christening, naming or welcoming ceremony or a party, you could put the date on the chocolate with the message, "Not to be eaten until 27.08.10... [or whatever your date is] - Lucy May Greene's Christening at St Peter's Church, 10am, see you there!" If your cards'/chocolate announcements' recipients show self-control and don't scoff the chocolate down straight away, they will be eyeing it every day sitting there on the kitchen bench until your newborn's Christening service date! What a way to remind people, he he he.
Here is another thought. If you prefer to have a photograph on the personalised chocolates wrappers, you could get a professional photo set taken by a photo service and have that printed onto your chocolate baby-announcements. This would be a nice way to show off your new family to all your friends and relatives, while making it a fun thing for their kids to receive.
All in all it would be one way to celebrate, in China they give mooncakes to each other as a celebratory gift on the occasions of newborn babies, engagements, weddings, etc... In Australia, NZ, the UK and the US we would probably choose chocolates over mooncakes, so here's a sweet way to do it, tailored specially to your baby or your family making it a unique and fun way to announce the baby's arrival!
Don't forget that mothers like chocolate too, so reserve a few for her, Dads, if it's you who's doing the ordering! ;)