The conventions are the distraction, the diversion..
It would be tough to get in there but the media would sure be a lure for a terrorist trying to maximize the news coverage of our losses. But then, for coverage the Athens games are also a good lure.
Al Qaeda always seemed to prefer to strike what it had failed to get before, though.
So, here's a game: if you were one of the terrorists, what would you hit?
Remember the rules: Kill randomly. Surprise. Get the media in attendance. Symbolism is ok, but terror is the desired end. Embarrass the leadership. Use all the advantages your arab/muslim ethnicity entitles you to in escaping scrutiny.
My bets: New Jersey's rail line, or perhaps Amtrak. (If they blew up SEPTA would anyone notice?) Or, a reprise of the random shooter along the highways and byways.