While (some) drivers may be distracted by having full access to the user interface while driving - that wouldn't apply to a front-seat passenger.
As long as there is any function in the car that is not directly associated with driving - it will serve as a distraction - tuning a radio, adjusting the air conditioning, adjusting the seats, ...
I've owned several Toyota and Lexus cars with their navigation systems - and they imposed restrictions while the car was in motion - they wouldn't allow destinations to be entered by name or address through the screen. But they still allowed destinations to be entered by scrolling around the map and then selecting a point on the map - a process that actually required more effort to complete than using the normal interface! My overall experience with the lockout was that (at least for me) it isn't necessary - and actually creates more distractions - not less - because things that should be simple - are made much more complicated by forcing workarounds to the limitations being imposed.
Tesla's design for the model S shifts all of the controls to the touch screen (and hopefully voice command). If they implement a high-quality, easy-to-use interface for the display, they should be able to reduce the distractions for the driver - plus with the ability to display some of the information on the dashboard directly in front of the driver - that may also help to keep focus on the road. And, with their ability to easily and quickly distribute updates to their vehicles - they can (and hopefully will) continue to improve the functionality and usability, based on customer feedback and experience.
Another possibility is to use the new vision tracking technology that is starting to be introduced in some vehicles, which uses a camera to track the driver's eyes to detect when the driver might not be paying sufficient attention to the road (such as falling asleep).
What if this technology could be used to track where the user's attention was being spent - and if too much time was being spent looking away from the front of the vehicle while the car was in motion - then a warning could be provided - and possibly a temporary lockout of the screen - until the driver gets back to watching the road...
Not only would this ensure the driver was paying attention to the road ahead - this would also allow passengers full access to the user interface - at all times.