Two thoughts: 1) when I pointed out MT to my girl over the summer and said something to the effect of "that's that big tower you would've seen on the skyline going up a couple years ago" she had no idea what I was talking about, which backs up my hunch that oblivious folks stay oblivious even in the face of a big new tower. On the other hand 2) an awful lot of camera shots from Fenway Park this coming season are going to show this thing looming real large, in which case the masses will truly be well aware.
I also think towers in Downtown kind of get lost/blend in. The Back Bay skyline is much more iconic and has been pretty much the Pru + Hancock with the minor addition of r2d2 (and of course the old Hancock). This will be pretty... noticeable and a pretty big change when looking at Boston and its skyline.