I think it would be extremely difficult to get to Bush unless they are scheming to use a SAM and attack the motorcade or out in the open somewhere. I would assume that any location would be swept pristine by LEOs before any Presidential presence.
I guess we can't underestimate the enemy, but I would be more worried about Tony Blair's safety at this point.
What about from a ship? What if a ship were two bows (approx mile) out could they launch something that would hit the convention? I had dreamn't the other day that two bows = two ships. I saw two big ships under a high bridge. The bridge was like an arch. I am wondering if the arch is the trajectory path of whatever they have. The sketch had an arch but with a plane. I am just trying to rack my brain here.