This is interesting:
Canon has also evolved the sensor technology, reworking the internals of each pixel on the way to producing what is promised to be a considerably less noisy sensor than its predecessor (and the 5D Mark II was already pretty good in this respect). Westfall indicates that at moderate-to-high ISO sensitivities, the 5D Mark III has about a two-stop advantage over the 5D Mark II. As an example, he says, an ISO 3200 picture shot with the new model has roughly the dynamic range, noise and other characteristics of an ISO 800 picture shot with its predecessor.
He emphasizes that the bulk of the higher ISO image quality improvement comes from enhancements to the image sensor itself, as opposed to heavier-duty noise reduction being applied once the picture has been converted from analog to digital form. The only camera from Canon that will beat the image quality of the 5D Mark III at higher sensitivities, says Westfall, is the EOS-1D X.