I was having the same problem, but seem to have found a workaround without having to buy any special products. First off I think there is clearly a bug, so submit feedback to Apple to get them to fix it.
As for the solution, what I've found is that voice control launches and the mic is disabled when I plug the mic cable into the phone jack AND the other end of the mic cable is plugged into something that is POWERED (i.e. your car stereo, powered external computer speakers). I have a BMW with an AUX port in my armrest. I can either unplug the 3.5mm cable from the armrest before I plug the other end of the cable into the iPhone (kind of a pain), or I simply plug the mic into the iPhone BEFORE I power on the car. It's so sensitive in my car that even if I just put the key fob into the dash, the voice control will turn on when I plug the 3.5mm cable in. So far in two weeks I've only had this process fail once, and couldn't figure out why.
I've seen people post that the cable makes a difference, but I'm not convinced. I did buy a cable at BestBuy made by Dynex for $5. It was the non fancy one that's back in the home theater section, not the $20 one in the iPod section that has gold plated tips.
Just to be clear, if my car is powered on, it's a guarantee that voice dial activates and the mic is deactivated.