To continue with tech issues, the Energy Blog has a post up about Altairnano's batteries, which have demonstrated long lifespans, high power density, though slightly below-average energy density. This was a conscious compromise on Altairnano's, apparently, because the balance of power and energy is explicitly designed for the electric vehicle market.
Judging by Altairnano's technical documents, building a plug-in hybrid with a 100km range would take about 120kg of batteries or so (assuming 200wh per mile, or 120 per km), store 12 kwh of electricity and offer a maximum of power of 640 horsepower... which should be enough for most applications (I would hope!)
Given that these batteries could be charged by a much, much smaller engine than normal, that's probably a net weight saving, even with the additional batteries. Weight saving = fuel efficiency.
I can't seem to find details on what the extra cost would be yet, but their docs say these batteries are extremely fast-charging, and have been tested to at least 9,000 deep-discharge cycles, meaning a lifespan of something like 24 years - long enough, in any case. Hell, a car maker could offer you a steep discount to trade in your old car, keep the old batteries, and save the recycling/landfill costs.
I've written about Altairnano before, here and here.