I have actually thought of installing a backup beeper on my 02 prius. It seems that people dont always realize that you are going to back out of a parking space when your engine dosen't sound like what they expect, they just stand there behind your car... This is a problem where I work and I regularly have to beep my horn before backing up so the people hanging around in the parking lot realize I am going to move.
I have also put some thought to adding a speaker in the front to play the sound from the jetsons car while driving at low speeds in parking lots.... another time you have to be more observant in the prius because of people steping out in front because they don't hear you coming.