Well, I was certainly talking out my ass about Saturn's magnetic field being used as a source of power =D
However, thermionic nuclear power sources have been used for decades. If you could prevent yourself from being buried in sulfur, Io would have tremendous geothermal energy. To be more accurate, the systems of Jupiter and Saturn do indeed have a fair amount of power in different forms that can be tapped into. For example, the gravitational forces induce a lot of heat, especially in the moons close to the planets in orbit.