I teach indoor cycling, and taught class 2 days before I delivered my second child. Riding while pregnant is certainly do-able, and I'm convinced that being fit made delivery easier.

Re: max HR... As you said, the 140 figure is out of date. Staying aerobic (and cool) is key. The catch is that everyone crosses their anaerobic/lactate threshold at a different point. For some, it may be at 85% of MHR, for others at 92% and for still others, it might be at 78%. I believe the best way to know is to pay attention to your breathing. As long as you can comfortably breathe thru your nose, you should be alright. (BUT I'm not a doctor, so please please please confirm this with him/her!)

The hardest part for me early on was keeping my intensity moderate. I didn't *feel* different, so it took conscious effort to not push myself. I had to remember that if there was anything wrong with my child when he was born, I'd always have regrets. (I know it's kind of a morbid way to think about it, but it worked for me.)

I think the "no running" thing has to do with the ligaments, but I'm not sure.

Best of luck!