The only reason for Canon to make a 24-70mm f/4L IS as opposed to an f/2.8L IS is simply cost. It is much easier and cheaper to make an IS lens with a smaller aperture, by a multiple of the cost of redesigning the 24-70mm f2.8 II lens (not twice as cheap but one eighth or one-tenth as cheap or some other fraction like that).
Everybody on CR moans about how pricey new L lenses are - so Canon are finally listening.
Daniel Flather + Unfocused have it right between them:
24-70mm f/4L $800,
24-70mm f/4L IS $1000
Both of these will appeal to 6D buyers (and 6D buyers will outnumber 5D3 owners by a similar factor as Rebel owners outnumber 7D owners).
Remember, the sharpest zoom lens that Canon ever made with IS was their 1st ever 4-stop lens: the 70-200mm f/4 L IS USM lens @ 85mm is sharper in the center than the newer 70-200mm f/2.8L IS II according to multiple reviews (and noticeably sharper than the non-IS version btw).
For 6D owners with 11 AF points and only 1 x cross-type center point, this f4 IS lens is perfectly suited (and as Unfocused says it will become the kit lens - just as the 24-105 f/4L IS was for the 5D2)
Finally, Canon may even release a 24-70mm f/2.8L IS lens in late-2013, but it will cost probably $2,700 or $2,800, so will only appeal to the comparatively fewer shooters that own a 5D3 or 1DX body.
edit: If I were Canon Inc. I would want a new affordable standard zoom for 6D owners, to be released in early-2013, one with better IQ than the current 24-105mm model whose focal length is too close to telephoto zooms.