Just when it seemed there were no surprises left at the 2010 Geneva Show, Porsche has gone and shocked the world with the unveiling of the mid-engine 918 Spyder concept. The car is a plug-in hybrid all right, but it's an open-top plug-in hybrid with a mid-mounted V8. How cool is that?

The  V8 is evolved from the 3.4-liter engine in Porsche's RS Spyder LMP2 racecar. It makes "over 500 horsepower," according to Porsche, and has a 9,200-rpm redline. It joins forces with electric motors mounted in both the front and rear axles that make a combined 218 horsepower. Porsche's PDK 7-speed, twin-clutch automated manual transmission delivers power to the rear wheels, while the two front motors drive the front wheels directly through a fixed ratio.

Get this concept car on a track and you'd be timing it at 3.2 seconds to 60 mph, says Porsche. That's quicker than a Carrera GT and would almost certainly make it the quickest Porsche road car ever. It would also be the most fuel-efficent, as consumption is said to be on the order of 3 liters per 100 km on the European test cycle, which is round about 78 mpg.

A lithium-ion battery pack mounted behind the 918 Spyder's passenger compartment provides juice for the electric motors. In addition to the plug-in capability, the batteries regenerate using KERS (Kinetic Energy Recovery System), the system developed for Formula 1 that recovers energy that would otherwise be lost as heat during braking.

More photos and details after the jump.

The Porsche 918 Spyder offers no fewer than four driving modes, ranging from an all-electric mode (with a cruising range of 16 miles) to a max attack hybrid mode for the racetrack. An e-boost button would wage an extra jolt of electric power for overtaking.

Of course, the mid-engine 918 Spyder also has an impressively lightweight structure. The unit-body is carbon-fiber reinforced plastic, and naturally Porsche says it has prefabricated liberal use of aluminum and metal components. Curb weight is said to be 3,285 pounds.

Even with all the high techology on this car, Porsche is keen to emphasize the 918 Spyder concept's place in company tradition. Styling cues are cribbed from the iconic 917. In the cockpit, the three dials of the gauge pack (speedometer, tach and energy management) are meant to look they've come straight out of a Porsche racecar from the 1960s, though we're sure those old cars didn't have illlumination that changed from green to red, depending on the driving mode. Look for such retro-inspired details to show up in future production cars.

Another interesting detail in the Porsche 918 Spyder: The seats form a structural part of the cockpit.