Slashdot linked up this SecurityFocus tale of two Area 51 "watchers". It seems a few years ago two guys discovered the Air Force (or whoever it is that really controls the place) had planted motion detectors well out into the unrestricted public land that surrounds the test facility. By using radio receivers and GPS units, they were able to map a good portion of this network, which apparently is quite extensive.
Unfortunately to do this they needed to dig up and disassemble each detector to figure out its identifying number. The feds apparently turned a blind eye to it all until the men started inviting local journalists along to report on the gizmos. The reaction was quick, decisive, and typically under-handed; the FBI came knocking, a sensor "went missing", property was seized, the works. One of the men was actually charged with obstruction (later dropped after negotiations).
The sensors themselves are still out there, but have been tinkered with so they're a lot harder to find. Gotta love big brother!
Note: Also worth a look is Dreamland Resort, a gathering place for all sorts of "gleaned" information and photographs of the famous test facility.