Those liability issues are probably what is going to kill the concept. Sure, someone voluntarily removing himself from the gene pool by screwing around with the ultracapcitor on his 2012 Volt is one thing. Stupidty breeds as much as it kills.
But the second we have ol' Grandma Milly get into an accident that causes the ultracapcitor to discharge all at once and electrify the entire vehicle, thereby turning her into an electrified (and probably smoking hot and/or flaming) granny, they'll deem the vehicle a death trap.
To me, it seems on the same level as putting a 10,000 PSI tank of Hydrogen into the back of a car. HELLO, HELLO! THINK MCFLY, THINK! Lets put the equivilent of the Hindenburg on the back of every vehicle in America, really get that Hydrogen economy into full, explosive force!
To me, just based on that fact, and that we use just as many fossil fuels to create the freaking Hydrogen as it would take to power a gasoline ICE, make it a pipe dream. Of course, it also made a convienient way for the Administration and Big Bussiness to look like they were "getting green" in mid 00's.
Now at least GM realizes we aren't going to realistically wean ourselves off of gasoline for a long time, but can try to make a vehicle that significantly uses as little as possible. They were also smart on choosing E-Flex, since it could accept any sort of fuel, so long as the generator used it, whether it's gasoline, ethanol, biodiesel, regular diesel (I'd kill to have my S-10 be a diesel, I'd convert it to Bio in a heartbeat)...even a blasted Hydrogen fuel cell, if need be.
It just seems funny to me, the railroads have been using this concept (sans battery power) for the last 70 years, and the automotive industry is just now catching up.