As reported in Wired, the device TV-B-Gone (copy of website here) is like an anti-TV handgrenade. When activated, the device will send a minutes worth of different shut down signales to televisions in the vicinity. It knows most every major make of television.
The Wired piece recounts the story of a succesful field test at a local Best Buy, where all the televisons in the store turned off one after another after one click on the device.
This reminds of that geek fantasy, the cell-phone jammer, but obviously this device is much less harmful. After all you can't call the emergency line on your television.