Neat. Anyone have an order-of-magnitude idea if this could be used for stationkeeping on sats in Earth orbit or for attitude control in deep space missions? Just wondering if it produces enough torque to control a real spacecraft. IIRC, for most spacecraft fuel for attitude control is the limiting factor on mission duration, and I think in some cases (e.g., Kepler) it's the only expendable.
You are correct that fuel for attitude control is generally the limiting factor for spacecraft (useable) lifetime. Using solar sails for attitude control would be possible, I think, for spacecraft operating far enough away from a planetary atmosphere. Otherwise, drag from the sail would certainly overwhelm solar pressure. So, though it may be possible, I'm not sure how economical it would be to use for stationkeeping. I would be interested in seeing a trade study between electric propulsion (another low thrust over long duration type system) and solar sails. Solar sails would probably mass more and take up more volume than an equivalent EP system, but would not require nearly as much electrical power, which would reduce solar panel size and save some mass and volume there. Bottom line, though, is that with the state of solar sail tech right now, it'll be a while before anyone tries using solar sails in such a manner. Most solar sail applications I think you'll see is as the main propulsion system getting craft between planets. That's where the greatest benefit of low thrust, but long duration burns that don't require propellant or little to no electric power will be.
Could a spacecraft using this technique have virtually unlimited life? If you're solar powered and don't burn fuel, what limits lifetime-- dust on the solar arrays? Battery degradation?
Aside from propellant, the big limit is degradation of solar panel generation capacity (from atomic oxygen, radiation, etc) and battery capacity that also degrades over time. Moving parts (like momentum or reaction wheels that control spacecraft pointing) can also wear out. Space is hard on lubricants. Then there are things that can ruin your day like computer errors from radiation effects (like the Galaxy 15 spacecraft) and impacts from space debris. So while eliminating the need for attitude control propellant will improve spacecraft life and is a worthwhile endeavor, don't count on getting unlimited spacecraft life any time soon.
"No matter where you go, there you are..." -- Buckaroo Banzai