OK, so I had a little more time to play with my iPhone 4S and the Nox. If you guys wanna use Siri over Bluetooth, just hold the Home button on the phone until Siri prompts. You'll notice that the car will enable Bluetooth voice and dial your own phone number. At this point you'll have a permanent connection to your phone. Depending on the tasks, you might have to tap the mic icon on your phone display, but generally speaking, you can use voice commands just fine.
You can tell Siri to play a song from you collection, but unfortunately that will not play over the car speakers. I can have her reading out SMS, even replay, tell me the weather, book appointments, etc.. Still Siri has very limited use in Canada, and most of the answers I get are " Sorry, I can't @&%&^%& in Canada.
From the steering wheel button, you can only make one command, and if it's not a phone call, the system will hang-up. Hopefully GM will release an update that will support the use of the steering wheel buttons a little better on the smart phones. Problem is that the primary function of the call button on the steering wheel is to access OnStar, and I hate having to say Bluetooth -> Voice -> then the command, every time I wanna use my smartphone. I had the same problem on the Android. Considering the car has another call button on the rear-view mirror for direct OnStar use, I wish GM will priorities the steering-wheel button for Bluetooth.
P.S. Chevrolet Customer Svc please note the bold text