Autopia has a decent write up about a problem with electric vehicles. Odd, I could swear we just discussed this issue over lunch.
My idea is similar to the Stanford grad students' use a device to make the car make noise. Nobody suggested pulling a few cards out of mom's bridge deck and putting them in the spokes of the wheels.
I think using the sound of internal combustion engines is boring. Why not have some fun with it and give it a Pac Man sound effect, play the Super Mario Brothers Song, Jaws theme, Road Runner sounds, Flintstones car sounds, the possibilities are only limited by your imagination and intellectual property lawyers.
One idea that I think would be pretty cool is to put a song into a speed aware device. The device would play the song quickly or slowly depending on the vehicle's speed.