They recently tested the Canon 1D Mark IV and concluded the following:
In essence, the Canon 1D Mark IV is the most advanced, lowest read noise, highest per pixel sensitivity cameras they've ever tested. That's an amazing conclusion and one that is quite exciting for 1D Mark IV owners.The Canon 1D Mark IV sensor sets new performance standards, including higher sensitivity per pixel (a combination of filter transmittance times effective fill factor times quantum efficiency), lower read noise, and lower fixed pattern noise. Hopefully the new technology that went into this sensor will makes its way into other cameras as well. That will enable a new era in performance, especially for high ISO / low light photography.