I just downloaded the high ISO images from DPR. All the images were taken at 3EV, so they are real low light images.
On my screen, I could see the noise at ISO 102400, but the 8.5 X 11 prints were sharp and clean. I then printed 3 more, 51200, 25,600, 12800, and they also suprised me. I could show any of these prints to someone, and they would not suspect that they were high ISO unless they had a magnifier.
These were jpegs with no additional processing by me, I would expect RAW to be better.
I certainly plan to use the ISO 25600 freely whenever I absolutely need it in low light for a fast shutter speed.
Canon has done some real work on the blacks, I can move the black slider in LR4 all the way to the right without seeing any banding.
No kidding. I think this is the test you're referring to:
I'm trying to be as unbiased as I can, but I had a very hard time picking up on any noticeable noise up to ISO 25,600. Only at 102,400 does it become objectionable. I'm starting to suspect that my eyes are bad or my monitor is broken Like you said, the noise in the blacks is almost non-existent. It looks like the native ISO range is a legit representation of the MKIII's true low-light capabilities.
Kudos to Canon for creating a high-speed, low-light monster. I didn't think I'd ever say that about any 5D