pedro ... everyone's definition of acceptable "clean" ISO is different, and often can be off by as much as 1 or 2 stops. I know people that are comfortable shooting the 5D2 at ISO 3200, others that refuse to shoot above ISO 800. personally, I find ISO 1600 is pretty much as high as I prefer to go in most circumstances, but ISO 3200 can work with some noise reduction applied and downres-ing.
If the 5D Mark III can deliver noise-free 1600 and clean 3200, a gain of 1 full stop, I'd be pretty darn happy with that. I'm sure others will be pickier. frankly, I think a 2-stop gain is asking for miracles in technology, which 3 years is not quite enough for.