Just noticed this in the 5DIII's promo video!
EF 24-70mm F2.8 IS II USM, could it be a typo?
There's also an IS on the 16-35mm!
It is most definitely a typo as there is no 24 - 70 f2.8 IS version 1. Not to mention 16 - 35 IS version 1. You need a version 1 before you can have a version 2.
If canon did in fact experiemnt with making a prototype 24 - 70 f2.8 IS as is suspected, and chose not to release it because of image quality compromizes, size and weight, I wonder if they ever will release such a lens for those who would gladly take slightly reduced IQ and greater weight for the IS. I suppose with the ISO capability of the 5D3, Canon figures you'll never need IS (for stills at least) because you can practically shoot in the dark with ISO 51200.