I like mothership's idea of just saying baby's due around Christmas. Then, people don't get all anxious for you. You will be anxious enough.
I went to a friend's party today, and almost told a few friends, but didn't. Every time I went to say something, someone else would enter the conversation and it just never felt right. The only people who know are my husband, my mom (whom is really like my best friend) and my dad (because she won't be able to keep a secret this huge from him).
My theory on telling vs. not telling is this. Tell whomever and whenever it is in your heart to tell, and if you don't think you could ever "untell" this person, then wait to tell them until later. Having gone through a pregnancy loss before, I really found it helpful to have certain people support us with what we were going through (can't imagine if my close friends hadn't have known), but at the same time EVERYONE knew I was pregnant with that baby and it was a little hard to tell all those people about the loss. Unless you have a job that would be jepordized by telling, there are only personal reasons not to tell. Maybe you and your partner want to keep it a secret just for the two of you, or maybe you just want to wait until the first ultrasound, etc. It's completely up to you. I always wind up telling people when I don't really expect to do it. It just kind of happens, because it feels "right".