Luigi just sent me the link to site of the "First Desktop Cloaking Device": Stealth Switch.

StealthSwitch™ is a microprocessor based computer privacy device. StealthSwitch™ uses patent pending technology to instantly and completely hide applications with a press of the footswitch. The applications are not just minimized…they are made completely invisible. No more minimizing applications, turning off the monitor, or re-booting when someone enters your office or cubicle. With a simple click of the foot switch, you can instantly hide the current window, hide all open windows, or hide all open windows except certain windows.

But, as always, a demo is better than a thousand words (click on the image to go to the demo):