Not too many details are available yet: it has a 1kg tank of hydrogen and a whopping 6kW (8HP) fuel cell that give it a range of 240 miles, and a top speed of 50mph. It weighs just 350kg (~771 pounds), owing mostly to the carbon fiber chassis. They are looking to produce 10 next year, and ~50 the year after that, with leases available in 2012, for £200 ($315) per month, including the hydrogen fuel. Key to their design is efficient regenerative braking (50% returned energy) using some ultracapacitors.
It looks like they are using hub motors in each wheel, and carbon fiber for the chassis. It does not appear to be on their web page (yet), but they plan to release their plans so that anybody can build one. I really like this idea of sharing the design information. The aerodynamics seem to be worked out pretty well — the range of the car with just 1kg of hydrogen is proof of this! The one (possible) snag is whether or not the side windows are operable — I guess toll roads can be handled with a wireless unit, though. There is an intake grill, which may be used for ventilating the car.
I have my concerns with hydrogen — you can either get it from processing natural gas (which is obviously not very “green”) — or you need to use renewable energy (electricity from solar/wind/tidal/wave/biomass, etc.) to make the hydrogen. There is no infrastructure for hydrogen, and this is a nontrivial hurdle. But with this Riversimple car, they provide the hydrogen as part of the lease. So this design concept works if you are located close to where they will have service.