That is a good point. However, both Toyota and Honda have over engineered their vehicles to sustain a lot of damage all the while rendering the high voltage electric system almost harmless. In addition the the usual fail safe relays that Bob mentioned, the batteries are protected by an extra strong structure/shell that in all likelyhood is the strongest area in the whole vehicle.
The curious aspect of all of this is that the traditional gas tank in these hybrids is far more vulnerable to a serious accident than the battery system plus the high voltage lines. I just wish the gas tank was as well protected as the high voltage circuit.

You'll also notice that most (if not all) emergency first responders have been receiving training on how to deal with hybrid vehicles involved in collisions. The high voltage lines are all painted (or sleeved) in orange color and designed to be noticed. In my opinion, the hybrid industry has done an excellent job and their vehicles are widstanding the scrutiny. In the case on the HCH, it received the highest safety ratings from numerous agencies which is pretty impressive safety record for a compact car. As a mid size vehicle the Prius has also been crashed many times and also proven to be an extremely safe vehicle.

At least on my side of the woods, the government agencies have commissioned exhaustive testing of these hybrids before adding them to their fleets and IIRC they have found the vehicles to emit negligible amounts of EMF... so little that it was hardly a health concern giving the unions little grounds to complain.

Cheers;

MSantos