Certainly iPhones, Pads and etc. work as well as any memory stick with music. The USB ports are for input, not output. Doubt if a mouse would work.
I saw a thread somewhere where someone used a mouse and it actually popped a cursor on the screen. I haven't tried it myself to verify though.
Various threads have collected information. USB ethernet adapters do not work. USB hard drives do work (at least my low-power 500GB 2.5" drive does) as do (obviously) USB sticks and the like. The earlier reply notes that mouse works but keyboard does not (although it may have been the manufacturer). - - - Updated - - - If the keyboard does work, I'd be curious to see if ctrl-alt-f1 works.
If I plug in my Leap Motion controller, I wonder if I could drive the Model S with hand gestures? "Hey Mom, why is that man waving his hands?"
Only had a Mac USB keyboard and an ancient logitech mouse to play around with, but seems that typing in text fields works fine, most other keystrokes are ignored. Hitting control-option-f1 seemed to immediately darken the touchscreen, but that was about all, it came back on in a few seconds. Mouse behavior was more than I was expecting, the scroll wheel actually worked for zooming in / out in the nav app! The cursor was indicated as a small blue dot.
I posted a detailed description of my successes in this thread, including pictures and videos of the mouse and keyboard in action.