Everyone needs to keep in mind, these are not yet posted as official results. Assuming they ARE official results, something else to keep in mind:
A DR of 12.8 is the Print DR. As everyone here who has read the debates about DXO before, particularly from Neuro and myself, Print DR statistics from DXO are very misleading. These numbers are not "impossible" like the D800's 14.4, however to remain objective and consistent in my argument:
Just as you couldn't actually capture a scene with 14.4 literal stops of DR with a D800 IN-CAMERA, neither will anyone be able to capture a scene with 12.8 literal stops of DR with a 1D X. For the exact same reasons, the PHYSICAL capabilities of the HARDWARE simply won't allow it. The hardware is rated by DXO's actual measurements, which fall under their Screen DR statistics.
My guess is that the 1D X will still have 11.something stops of real-world HARDWARE DR. The 12.8 stops is only something that might potentially be possible with the right kind of scaling algorithm, and all it will do is allow you to utilize a little more headroom that is normally consumed by noise in a 100% image (however at the tradeoff of detail and resolution...potentially a LOT of detail and resolution, since it required downscaling). The 12.8 stop DR rating tells you about what SOFTWARE can do if it normalizes (bins) noise, but it does not tell you anything about the physical capabilities of the hardware. (Although if 12.8 stops really is the Print DR, it sounds like the QUALITY of the 1D X's noise is really quite good.)
Just to put things in perspective, and maintain a level playing field with consistent arguments: Screen DR tells you about the hardware (and we don't know this yet, probably won't until DXO actually posts the 1D X results on their site.) Print DR tells you about how clever DXO's testing software is and how capable it is at normalizing noise when downscaling.