Thanks, all. I did ask my fill-in doc why she thinks I shouldn't ride at all from the moment the pregnancy begins, and she said it's for safety, it's a matter of balance, and I'll probably be too uncomfortable to want to do it anyway. I understand that at 8 1/2 months along, I'm probably not going to want to train for a century ride (heck, I don't do that now!), but I figured I'd still want to ride for at least the first half of my pregnancy. I ride 30-40 miles most of the time, and I try and get in at least one or two good rides a week. I can't imagine not doing that for 9 months. I'm guessing that Dr. Fill-In doesn't ride a bike and, while very kind, probably doesn't understand why I'd like to ride while I still can, instead of just switching my exercise regimen to maternity water ballet classes at the gym.
I figure that I'll call my friend's sister, who is an OB in a different state, and chat with her about it, and also ask my general doctor, who I know is a cyclist because he has pictures of himself riding in the office.

Between gathering various opinions, I'll at least be able to get a better perspective on what other docs have to say about it.