First of all, congrats!! Your first??
I posted the same question a while ago - I'll see if I can find it. I think it was called "Working Out While Pregnant".
Basically, the consensus was:
1. Get doctor's advice
2. Yoga - make sure the trainer is certified/qualified in dealing with pregnant women.
3. Don't do anything radical - as BTB said walking and light weights.
4. There are some exercises/positions that are not appropriate for pregnant women i.e. flat bench - cuts off circulation/oxygen to the uterus.
5. Her joint flexibility will increase due to hormonal changes preparing her body for birth. So don't over do it and get injured.
6. Research for yourselves and take the slow gradual approach.
If I remember any more - I'll let you know.