How about swimming? that works all the muscles (nice and gently) and helps with flexibility etc especially if you are hoping for a natural birth (says she, who had a caesarean!).
I agree that pregnancy yoga is excellent, but long walks and swimming are effective and time-flexible too.
7 months after having my little boy I am FINALLY almost back to the pre-pregnancy me - it is harder work afterwards, but I think that's more to do with having to juggle work, husband, baby, exercise, etc.
I too have heard horrors about stomach muscles - indeed one of my friends can fit her fist between her stomach muscles and her baby is just over a year old... yuck! What I would say though, is that it is normal to be aware of a gap for a few months after birth - especially if you have quite a lean stomach. I probably still have about a 1cm gap below my belly button, but it is going :o)
Re: Pilates-style exercises, I had a personal trainer to get me back on the road to good training during the summer, and he advised me to do a lot of core work - typical Pilates exercises for the lower stomach area and pelvic floor, but told me NOT to do any crunches or sit ups until a year after birth. His reasoning is that the muscles need time to get back into place, and if you exercise them too early you get the pot belly effect and it is also bad for your back.
So...! Sorry for the essay, but I hope it helps in some way.
Hegs x