If you thought that Siri would get some special treatment in California because it was born in Cupertino California, you're wrong because it is illegal to use Siri while driving in California. Its possible that using Siri when driving is going to be illegal in many other states as well as countries.
San Jose Police Lt. Chris Monahan explains:
It’s legal to talk to Siri, as long as the phone’s not in your hand. But if you have to push the phone to activate her, or if you ask for directions and she puts them up on her screen for you to read, then California’s hands-free law says your’re breaking the law.Regardless of the fact that Siri can be activated without actually having to touch the button through a wired or Bluetooth headset, reading content on the screen will still be considered a violation. For example if you ask Siri for directions, it would launch the Maps app and display directions on screen.
The law in this case states that if a device is held in the hand while driving, it is a clear violation of the law however one could argue that the iPhone 4S serves the purpose of a GPS device as well and when mounted in the car, its no different than a conventional GPS.
The law and technology have still a long way to go before they catch up with each other. Meanwhile, its always better to avoid a ticket, so keep your interaction with Siri when driving at a minimum.