Last night I saw a documentary in the History Channel which dealt with military vehicles (hummer-like) using diesel and electric motors with Li Ion batteries, one for each wheel!. Diesel helps charging the batteries while driving. They also showed how a concept car powered by a couple thousand of notebook computer batteries (Li Ion too) had better acceleration in the 1/4 mile than a V-10 Dodge Viper! BTW, Believe it or not, I almost got ran over by an electrical test vehicle from GM I think back in the mid 1990s in Boston, it was going real fast! The documentary also showed a GM concept car powered by hydrogen/oxygen. I think that special is called from Tactics to Practice or something rather (since I saw it in spanish, from my local cable operator).
My wife's uncle works for a company that makes Ultra-Capacitors. They are using them in some hybrids and regeneration systems on vehicles like buses. The UCs are more efficient at accepting the regenerated power during vehicle braking than batteries and weigh less to boot.
Not really intended for supplying power while driving along for any distance, more to contain and deliver sudden large charges.
He told me about some of the buses and what a big difference that using the electric motor off the line makes. Saves a ton of fuel and emissions and makes the bus take off more like a passenger car. They are doing the same thing on some Mercedes sports cars. I guess they really rocket off the line when assisted by the electric motor.