Misplacing keys and cellphones is a far to common occurrence than it should be, especially with the level of technology available to solve a problem like this. I Call Me is an idea for a two-way calling system to locate keys or the cellphone.
The device works with your current phone. It is a small fob that goes on the keychain. If you misplace your keys, a call can be made from your cellphone to the fob and it will begin chirping and blinking. If the cellphone is lost, there is a button on the fob that will call the cellphone. Lose both? No help there, buddy. Maybe some tape?
This idea isn't flawless — if the fob acts as another cellphone, then over-the-air monthly charges would have to exist among other issues. How about some Bluetooth action?