I'm thinking this could put significant downward pressure on the original version of Canon's lens.
Again, not necessarily: The distance to the mk2 is very large, and the Tamron's 24-70's af is not good as Canon's which might make a decisive difference to event photogs.
The AF comparisons I've seen puts Tamrons AF speed/accuracy very close to Canon 24-70 mkI, and certainly "good enough" for event photography. The MTF test from Photozone is actually a little better than I was expecting, and the great center sharpness at f/2.8 from about 35mm to 50mm bodes well for that critial range where 35mm and 50mm primes usually rule.
I think a sensible event photog could EASILY produce fabulous photos for clients with this Tamron, and with the money saved from not buying the Canon 24-70 MkII, a nice, fast prime lens could be added to the bag. And when lights go low during receptions, the IS on Tamron could spank the Canon 24-70 II no matter how much better the MTF may look in lab.
I hope the stiff competition being put up by Tamron for lenses and Nikon for bodies will at least give pause to those Canon Exec's who seem hell-bent ONLY on pricing their gear as high as possible as they can get away with.