The impression I got from the multitude of videos and demos that I have seen is that many of the basic touchscreen functions like audio and phone will be voice activated as well. In addition, the steering wheel controls can do quite a bit.
Now, onto Siri. Siri works by taking your "voice command" or request, transmitting it to Apple, processing it, then sending back a response. No internet connection and Siri is useless. Slow internet connection and Siri is tediously slow, so you will abandon your efforts often and revert to manual control, thus making Siri a worthless endeavor. To get Siri ported to the Model S, it would require quite a bit of software support to allow for control within the car...meaning software integration from multiple sources (Tesla, Apple). Siri is proprietary, so no real way around that. Apple controls this on the iPhone by providing isolation of it's "subsystems" via API's. As Siri evolves, SDK's will allow developers to access the Siri API, which will then be allowed to access other API's such as audio and video, or access apps. I don't know if we really want Siri in the Model S. Access to Siri via the bluetooth to allow control of the iPhone is another deal entirely, however, and I am hopeful that we can do that. My GMC allows this and makes dialing or listening to emails or text messages very simple while driving.