"Where does US law stand on accessing actual Satnav units - either built in, or third-party (suckered to the windscreen)?"
US law says nothing, these are all state laws. But, indeed, this is a good reason I opposed anti cell phone laws. People'd look at me like I'm nuts and I'd point out, well, no, I don't want people on their phones while driving. But, the right way to handle it would have been to make it clear that dicking around with electronics while driving will be considered distracted driving and enforced as such, rather than a narrow law that makes cell phone use illegal but probably not even tablets. Oh well.
That said, sounds like he was not using Google Nav -- he was using Google *Maps*. Important difference, Nav would have just been sitting there and reading directions, Maps he would have been dragging the map around while (probably) insisting it didn't affect his driving at all.
"Now which do you think is more distracting? Using your phone as GPS, or trying to unfold the correct portion of a giant map and read the tiny writing to figure out where the hell you are? Personally, I find that I'm far more focused on the road when using the GPS on my phone."
Probably the touch screen, because your dragging a little picture around on a little screen instead of having a persistent printed map. That said, both are distracted driving and you should get a fat fine for both. Plan your route ahead, and pull over and stop if you need to seriously work over your map (electronic or paper.)
So, I don't recommend driving distracted in California. I took I-5 north from San Diego to Los Angeles about 10 or 15 years ago... well, through San Diego, the traffic slowed to about 60 in the 30MPH zones. Up I-5, the slow lane was doing about 100MPH (people were passing me, because my Chevy Celebrity would top out at about 95-100MPH unless it was very flat), middle lane about 110MPH (I could hear cars bounce off their speed governor every so often, which on some cars is at 111 or 112MPH), and faster lanes appeared to be pushing 140MPH. In LA, in heavier traffic it alternated between 50MPH or so and gridlocked at a dead stop; lighter traffic people just drove as fast as physically possible as near as I can tell. I seriously doubt you'd be hassled IF you were messing with a map stopped in gridlock, the rest of the time it would have been SERIOUSLY unsafe to even think about it.