The Cobra PhoneTag ($60) does exactly that, using a Bluetooth connection between the included dongle and your iPhone, Android device, or BlackBerry to let you know if your tagged item has left the vicinity. It immediately sends you an email or text message with GPS coordinates when it happens, and works the other way around, too, letting you “ring” your smartphone from the PhoneTag. Just don’t place your phone in the bag that’s got the PhoneTag on it, or you’ll be completely screwed.
~~~This is probably the single greatest creation EVER!!!!!!