I'm all for this. But I've heard that the filaments will generate voltage in then ionosphere. Keep me on this list.
As a tether passes through the earth's magnetic field it gathers ions, simultaneously generating a thrust vector normal to the tether, slowing the satellite it's attached to. Think about how electric motors work. This thrust vector could be used to deorbit old satellites & space junk.
If a current (provided by solar panels) is applied to the tether it will generate a thrust vector in the other direction, speeding the satellite up. This could be used to boost a satellite to a higher orbit. The neat thing about this kind of propulsion is that you don't have to worry about running out of fuel- as long as your solar panels keep working you will continue to have propulsion.
Check out the papers at www.tethers.com if you want it explained better.