The Wireless Camera Hunter is a handheld, video Walkman-size device that automatically detects and shows (it's got a 2.5-inch screen) video transmissions in the 900MHz to 2.52 GHz range--the range used by nanny-cams as well as "secret spy" surveillance cameras. (Several online stores stock a wide variety of this sort of technology. It's worth checking out if only for the creepy feeling you get when you realize that people--perhaps even your neighbors--actually buy and use this stuff. Search on "spy camera.") As far as I can tell, the Hunter (and, presumably, the cameras) have no audio channel--these are silent movies.
This ought to be disturbing, I suppose, but I can't get excited about it. Yes, my baby monitor is now compromised, but most of the time it shows a soundless video feed of an empty crib. You might as well be using your $500 device to stare at a really tiny B&W photograph of a stuffed bear and a blanket with Gerber stains on it. Now, I've heard stories of people with audio baby monitors eavesdropping on fights held in front of neighbors' transmitters (when the two devices inadvertently share a channel). That's disturbing, I admit. But most of the time, most of us aren't fighting--we're just getting along with our quotidian concerns, talking about whether to rotate the tires and whose turn it is to cook.
And so we come to our primary protection in an age of increasing vulnerability to surveillance: The spy's desire to listen to something interesting. By which I mean: Most of us (I include myself firmly in this category) are just too uninteresting to be worth overhearing. As Josef Stalin might have put it, "The boring have nothing to fear."