I have an '07 BMW and when the Bluetooth is connected and I press and hold the button to activate Siri it will work through my cars speakers. I never use it though because while driving, the road noise makes it almost impossible for Siri to accurately understand me. So when they do implement this into cars down the line I'm sure they'll take this into consideration and will have some sort of noise cancelling function for background/road noise.
I think the OP is asking about pushing the bluetooth button on your steering wheel and it activating siri.
All bluetooth cars with a mike can work with siri if you push the siri button/hold home on your phone.
my 2012 mazda3 works with it. I dont have any problems with the mic hearing what i say either. Maybe its your mic in your car?