I think this is an obvious replacement of the relatively old 28-135mm f/3.5-5.6 IS USM to go along with the 6D. I think it will definitely be smaller and lighter than the aforementioned lens (as well as the 24-105mm f/4L IS USM), and priced around $600 to compete with the Nikon 24-85mm f/3.5-4.5 VR. It would be the perfect kit lens for a first-FF DSLR buyer. It would definitely have better IQ and IS than the 1998.
Think about a 6D buyer looking at the currently lineup of FF standard zooms:
28-135mm $450 alone (big, heavy, old, fair IQ)
24-105mm $970 alone $800 kit (big, heavy, very good IQ, somewhat expensive)
24-70mm II $2300 alone (big, heavy, superb IQ, more expensive than the body)
...and that is all your currently-available new (not used) Canon options
Replacing the 28-135 makes total sense with the first entry-level FF coming out in a couple months. If the only option is 6D+24-105mm ($2900), it is more expensive than a D600+24-85mm ($2600), then Canon WILL lose customers, period. A more affordable is needed, and the 28-135mm is not the solution. A red ring and L in the name will elate potential customers (much better branding than Nikon's ED and gold ring designations).
EDIT: Price correction on 28-135mm