I think it's fair to guess that the f/5.6 AF limit of the 1D X was not an intentional crippling, but rather was done to ensure the AF was working at its best. Basically, they are trying to wean 1D series photographers off cheap lenses...
To be sure there's an engineering angle in this, but (as others have found and as I've stated) there is every reason to believe that the amount of light hitting the AF sensor, be it slightly more or slightly less, will correspondingly help or hinder the AF process. The sheer density of the new 1D X AF sensor might have something to do with this, as well (actually that was my first thought, in typing this up).
I don't think Canon would be so bold as to tell users of the lower-end cameras to give up their lenses - but they shouldn't need to, since with the 1D X the f/5.6 limit is apparently becoming standard. That signals no change for sub-1D series bodies, not evidence (to me) of a willingness to cripple them.