Who cares as long as it has stunning image quality? This is going to be Tamron's biggest issue by far. Every single one of their lenses, except for the 18-270mm & 60mm Macro, has performance that matches Canon lenses from the late 80's to early 90's for a price that's usually not much lower than the current ones.
You forgot the Tamron 17-50/2.8 non-VC version that optically perform comparably to the EF-S 17-55/2.8 USM IS (i.e. excellent) for a fraction of the price.
That's not a very encouraging MTF chart.
Comparing the MTF charts from the Tamron to the 24-70L I it looks like the Canon is going to have better sharpness.
Whaaat? I don't know how you
read the MTF charts, but looking at the ones provided by dilbert (thanks), wide open the Tamron will generally be similar or superior to be mark I (in particular in the corners and at 70mm). The bokeh is also poised to be great (as smirkypants pointed out), but we will have to wait for actual samples to be definitive.
If there are no AF issues, with VC I'd say we have a clear winner here (compared to the mark I). Compared to the mark II the story is different, but I could still see the Tamron be attractive, not the least because of the price and the VC. If build quality or the zoom/focus ring directions are of critical importance, then no, the Canon will clearly be the lens for you.