maybe some sort of electromagnet system could work?
you could have its on/off button on a remote attached on the bars.
ha, just though of a name: ejecto pedals!
hit the button and the magnets polarity reverses throwing you off the bike. (parachute not included). 😆
powering it could be a pain, maybe through some sort of device (similar to the phone charger) that generates power through pedalling?
hey theres another great idea right there that should go further than just the mobile phones/ipods… proper pedal power! plug in household appliances to your bike and power them by cycling! and what about pedal powered cycle lights?