General comment: If this rumor is true, it seems to me there won't be a 24-70IS at all?! Not that it would matter to me, a €3000+ price tag is not in my budget region for a standard zoom anymore.
What do facts have to do with anything, people don't want facts interfering with their opinions.
Most people of course are open to interfering facts - but in my experience it is often discovered that these facts, after due inspection, either aren't really proven/applicable or even magically validate the seemingly contradicting opinion as the exception to the rule :-p
The 24-70mm II uses I beleive (don't quote me on the precise number) a 68mm front element with an 82mm filter size. The 24-70mm f/2.8 IS used a 77.29mm front element!
So in simple layman's terms a Canon 24-70/2.8IS needs larger glass elements to be sharper than the 82mm Tamron 24-70vc, or does the Canon IS have another construction that is larger than Tamron VC?
I have to admit I don't quite understand it yet because I imagined the IS being build *around* the lens, but now it seems like the IS needs some glass "headroom", at least in the front element, to work because the angle of the lens changes slightly when IS is at work?