KiltedTim, the desire to communicate directly with Siri, rather than through the car's Bluetooth system, is because car Bluetooth systems pre-dating Siri actively control the phone for making calls only, not for talking to the phone itself. Since the pre-Siri car doesn't "know" about Siri, Playadude wishes to communicate with Siri directly through the phone. But he can't, because the iPhone's microphone is disabled because the car's Bluetooth is enabled.
One could disable Bluetooth, then talk with Siri normally, but then you lose Bluetooth for hands free calling while driving. It's not easy to connect/disconnect Bluetooth, especially unsafe to try to do while driving.
It seems a simple programming matter Apple could easily address. A code (say, a 4-digit "phone number") could be put in to the iOS as recognizing a user of a pre-Siri Bluetooth enabled car wishes to speak with Siri. The user simply makes a "phone call" using the code. The phone should then "connect" as if making a call but the user would be speaking to Siri directly. Pressing the hands free button on the car permits the driver to speak the 4-digit code, then say "Call". The iPhone could interpret this as a desire to speak to Siri, "connect" the speakerphone function as if completing/connecting a call, and issue the familiar beeps through the car's speaker system. The driver could then make requests of Siri directly. For cars enabled with voice-activated phone books, the user could then save the four-digit code as a contact, name it "Siri", and voila, one could "call" Siri using the car's existing Bluetooth system.
But before any of that can happen, the car must first have a way to "hold down the iPhone's home key" and activate Bluetooth as if it were making a call. By definition, pre-Siri Bluetooth car phones are designed to place hands free calls. The trick would be getting Siri to recognize a certain phone number as a request to "call" Siri herself, and for the iPhone to respond as if it were completing a call, only to Siri. I have this very problem with my 2008 Acura TL. Any ideas for a workaround?