The computer for the driving functions of the car and for the geeky gadgets are two different things. I hope and assume that on the MS these are completely separate, as they are on other cars, afaik. So the worst that could happen is issues with the geeky-stuff software.
This said (I am an IT security guy), if it is well done, there should be no way of transfering executable files to the car, or to somehow trigger execution of software on the car side, simply because the car listens only to specific functions that can be executed over Bluetooth. So even if your phone has malware, it would not go over to the car, and even less so to affect its driving functions.
Should. Because in this matter, you never know how creative some guys can be. Still, many people use Bluetooth in their cars today, but I've never heard of issues like this so far.