HI there - I am looking at buying a permanent magnet motor to use to generate power from a bicycle - the specs say it is a 12v dc motor and says "15A 5000 rpm" - does this mean that it needs to spin at 5000rpm to produce 15amps...what if it spins at a much much lower rpm?

First of all, if you are buying a permanent magnet motor and intend to use it to generate power from a bicycle, it isn't a motor, it's a generator. Secondly, a rating of 12 volts at 5000 RPM means that it would produce a voltage of 12 volts if it is spinning at that speed. At lower speeds the voltage would be proportionately lower. The ampere rating is the amount of electrical load it can safely handle without overheating or burning up. If it delivers a load that equals 12/R = 15A or less, it should be able to handle it. R being the resistance in Ohms of the load connected to it. If the load is an incandescent lamp, I would expect the light output to get dim when you slow down or stop, and be brightewst when the generator is spinning at the specified RPM.

I have heard of people using a similar hook-up to power a TV so that they can get their exercise and watch their program - as long as they keep pedaling - on an exercise bike, of course.

15 Amps at 12 volts is just about 1/4 horsepower, which sounds to me to be rather much for an average human being to deliver or maintain for any length of time, so I wouldn't think that much load was practical??