Primitive battery technology is holding back plug-in hybrid vehicle commercialization.  Current batteries just don't have a long enough range.  But researchers at Stanford have shown that battery capacity and lifetime can both be increased up to 10 times by substituting silicon nano-wires for carbon electrodes, according to Denis Drew here.  He has a link to the Stanford press release a year ago and suggests substantial subsidies to plug-in hybrid vehicles could be paid for by the reduced transfers to petro-states for imported oil.