The Economist does a good summary of ultracapacitors, saying "A new version of an old idea is threatening the battery industry." It first covers the XH-150 "Tribrid" covered in TreeHugger here, and then continues:
Eventually, the so-called ultracapacitors on which the XH-150 is based may supplant rather than merely supplement a car's batteries. And if that happens, a lot of other batteries may be for the chop, too. For it is possible that the long and expensive search for a better battery to power the brave, new, emission-free electrical world has been following the wrong trail." They then provide a technical explanation of the difference between batteries and capacitors, noting that:
The whole process of ion movement and chemical reaction [in batteries] is slower than the movement of electrons in a capacitor. Hence the different advantages of the two storage systems: capacitors give speed; batteries, endurance.
They then give a tip of the hat to our old friends at EEStor, "a Texan firm that has developed a capacitor it claims can store "very high" levels of energy using a special insulator called barium titanate rather than an electrolyte. Its "Electrical Energy Storage Units" will go into production later this year."
Or so they say. ::Economist