I slapped together a pedal generator this morning, using an old exercise bike that I bought for this eventual purpose, and a treadmill motor I bought on the weekend for such experimental purposes.
Friction drive, with the tire rubbing against the flywheel of the motor. Its a 130v permanent magnet motor, runs at about 400 rpm off of 12 volts, so it just happens that running friction drive against the tire gives you probably 500 rpm at a steady cycling speed.
I put a load on it (100w inverter, and a sub 100w fan), and a diode to prevent the battery from running the motor, and tested it to see how hard it was to keep the battery voltage steady.
That steady, easy pedaling speed would put out about 4 amps and hold the voltage around 12.6v. I'm out of shape, but if I got going fast enough, I could get it up past 14v, and about 8 amps.
It took a bit of screwing around, and my lungs got a good workout...but maybe now that I've combined a "doohickie" with exercise, I might be inclined to get some. :P