The S2's sensor is actually very large. Larger, in fact, than a standard 35mm frame, at 30 x 45 mm. That's 56% bigger. The S2 is essentially a medium format camera shoe-horned into a 35mm DSLR-sized body, and Leica is promoting this idea in its press releases.
The body contains a standard focal plane shutter, but the new lenses also have leaf shutters. allowing for very high flash sync speeds.
The CCD sensor is made by Kodak and the Maestro processing chip produces a very low amount of noise at very high ISOs. The body is acctually smaller than, a Nikon D3.