The same way you steer a car with normal power steering when the engine dies: you pull harder on the steering wheel. In both cases, you have power assisted steering, there's still a mechanical connection to the front wheels.
Actually, I would expect the Prius to be easier to steer without the power assist than normal cars. With the electric assist, if it shuts down, you should just be steering without the assist. In a nornal car, you have to fight against the power steering pump, and I know from experience that it's harder to steer than if you had the same car without power steering. It's not that horribly difficult, but it is more work than I'd want to do all the time.
Thanks for your reply. Somebody told me that Prius was pure "drive-by-wire" and that the steering was electric-only. That struck me as a major safety issue. What you say about the Prius probably being easier to steer than conventional power steering in a breakdown sounds good to me.
So far, the only paranoid "what happens on failure" issue I have at the moment is where you can't release the "park" setting if the system wont start up. Everything else (including the brakes, seems to have a failsafe system). In the case of not being able to release "park", a dolly is the only way to get the vehicle rolling.
BTW, does anyone know whether the "foot parking break" works the front wheels, or the rear ??