I encountered my first airport naked body scanner while flying out of California today, and of course I decided to "opt out" of the scan. You do this by telling the blue-shirted TSA agents that you simply wish to opt out of the body scanner. Here's what happened after that:
I actually did receive a pat-down once without having to "opt out" of the naked body scanners. It was at Sea-Tac and they didn't have the scanners yet, though I understand they are coming. Apparently, the TSA agent thought my clothes were too loosely fitting and decided I need a quick pat-down. It covered the extremities and my trunk—without the "back of the hand" crotch pat down.
I have only seen the naked body scanners with my own eyes in one place: the International terminals at SFO. At that time, I went through the machine, even though I felt weird doing so. Everything I'm reading suggests I should opt-out, if for no other reason than to stand up for my rights. I'll make sure to arrive extra early so the TSA agents have plenty of time to "screen me."