I know that this post is now "old" and I'm pretty certain you've had your baby without complication, however I'd like to toss in my two pennies about this if I may.
I, too, was faced with the same dilemma during my second pregnancy last year (2011 - had my DD in Dec). My hubby and I both ride sport bikes but he has access to his Dad's cruiser as well. I was quite comfortable riding my bike alone until my fifth month. I live in Niagara Falls. I've lived here all my life & know the ways to get around the busy areas and where quiet streets, etc are. I was careful. I stopped because it just wasn't comfortable anymore. However, I didn't give up the ride. Like Heidi, it was pretty much the only "alone" time I got to spend with my hubby. I rode on the back of his sport bike and the cruiser - on short distance rides - up until the weekend before the baby was born.
Other than low blood sugar (due to my diabetes), the baby was perfect and she is a very mellow and easy going baby (now nearly 10 mos). She loves being in the garage with Daddy while he's working on the bikes....even loves sitting on the bikes. The noise does not bother her at all (the vacuum is a different story though LOL).
I spoke to my OB about this earlier in my pregnancy. He explained that there are women who ride horseback until their due date. There is just as great a risk falling down the stairs at home as there is getting into a vehicle accident (regardless of what type). I completely understand where women are coming from who are against pg women riding motorcycles, however, like anything else, the decision is up to the woman and others have no right to criticize - but of course are entitle to their opinion.
I feel that, as long as a woman feels comfortable doing whatever it is she's doing; playing certain sports, riding horse, bike (street or pedal), etc - and she's being as safe as she can be about it, then go for it!
I am somewhere between 16-18 weeks right now and plan to do one last ride on my sport bike; about 20 min to drive it to my friends and park it in his garage until next season. My last ride.....short & sweet. Unless I go out with my hubby. We'll see.
So in short......don't stop living your life and doing what you enjoy doing. Your baby is more protected than what you think. Just be super careful. Accidents happen (and yes, I KNOW they can be prevented too) - it's a part of life and everything happens for a reason.
Ride on!!
Is it safe? 
I mean I know motorcycle riding has its risks alone, but there isn't really an increased risk while pregnant is there? My husband & I enjoy a 20-30 minute ride around town together once every other week (when my SD is at her mothers) and I would hate to give up our together time 
I know I'll have to give it up once my belly doesn't fit anymore, but for the time being it's safe, isn't it? We stay on smooth side roads and never go over 30 mph so I figure that the risk is the same as being in a car.
I have a scooter here, but since I can't quite fit into any pair of pants at this moment due to all the bloating, I had to wear dresses and of course, cannot ride my scooter to work. 
