[Taniwha]

Depends on a bunch of things - how hard are you pressing on the brakes? above a certain threshold you're going to exceed the max current of the electrical subsystems and the traditional brakes will kick in (you don't WANT to hit that truck do you) - also once the battery is full the regen system has nowhere to dump that energy, my Prius behaves slightly differently as it switches over from regen braking once the battery is full.

[serpix]

Prius and other Toyota hybrids actually burn gasoline going downhill. Try it. Put B and go downhill and observe the consumption, the engine is on and burning gas.