There have been a couple threads on this before. I will put forth a couple ideas and preface them with the note that I am not an automotive engineer, but have a degree in electronic engineering and am a very handy person when it comes to things electric,electronic or mechanical.
#1 thought: batteries, even modern ones are heavy. I will make an educated guess that they are heaver than the engine in the car as I have removed an small iron block engine, disassembled, took to a machine shop, reassembled and reinstalled without the use of an engine hoist or stand or any big tools to help me and todays engines are more aluminum than iron. My laptop is 3.5lbs and if I pull the battery out it falls over backwards because the battery is most of the weight. My point is the battery box and mounts will need to be strong to support that weight so that area is likely to survive the crash pretty well.
#2 the motors produce a lot or torque and therefore require a strong frame to hold them in place which should also help keep the car intact.
#3 other high voltage or high current systems, HID lights for example, have safetys built in that shut them down when an impact is detected.
#4 race cars are required to have kill switches as near to the battery as possible and within reach or rescue personal, is it possible that the hybrid and electric car makershave installed a kill switch somewhere?
#5 the GM EV1 has been around for years and made a lot of news but I don't ever remember seeing or hearing any news about accident issues with them, I would think one or more would have had an accident at one time or another.