Impractical. A GPS receiver in the downtube may not work as it wont get a good enough signal in a metal tube (not so for a carbon frame - but then I dont know the shielding characteristics of that). Also, the gps transmitter has to transmit in order for you to know where it is. Which means you need to hookup with some sort of wireless network provider. Would the end user be willing to pay a monthly subscription fee (like lo-jack) ?