I have a 2006 Toyota with Bluetooth, it would be nice if there was some functionality (or even a hack) that would convert the phone pickup or phone hangup buttons to activate Siri.
My 2011 Nissan has the same issue - the phone button on the steering wheel simply activates a menu on the in-dash display, rather than activating Siri. I've read that certain other cars (I believe a Prius was mentioned)
will activate Siri if you hold down the steering wheel button. Sadly, mine still doesn't.
My car came with Bluetooth phone and audio integration, and I think that's sufficent for most of the functionality of fancy 'eye free' cars.
Agreed. It doesn't make any sense to me why this "eyes free" mode requires any integration with the vehicle. When I hold down the home button, Siri automatically utilizes the bluetooth audio system in my car. Works fine (though the steering wheel button as mentioned above would be better).
One of my biggest gripes with Siri in general is the fact that it often displays answers on screen without reading them back to you aloud. Totally defeats the purpose of any "eyes-free" mode while driving. Example: I say "where's my wife" and Siri uses Find My Friends to locate her, displaying a map on the screen. FMF already knows the address and/or place name, so just read that aloud to me!

Again, correcting this deficiency requires no integration with the car.
The other issue is that there's no way to unlock the phone via voice. If I'm alone in the car, I should be able to choose to use an "insecure" method of unlocking by speaking a passphrase aloud. Having Siri refuse to read new messages because the phone is locked while I'm hurtling down the highway defeats the purpose of eyes (and hands) free.
On the other hand, mirroring the phone display on the car's in-dash display would be awesome. Hopefully Apple can work out an open standard to do this so that any car manufacturer can add the capability without having to collaborate directly with Apple. Otherwise we'll only see it on a few select models.