So, given that every terrorist has a motive, the question is one of what that motive is.
Either they are trying to achieve their own specific ends (murder or destruction, usually of specific target(s))
send a message to a small group (again, specific targets which would be meaningful to those the message is aimed at, if not the general public)
destabilize a government or economy--in which case target selection would reflect their aims by going after specific political figures or key personnel, or be of specific economic importance
or destabilize a population by causing widespread fear--in which case a target might (or might not be) high profile in that it would be well attended by media, but the apparent randomness of the attack is the key to causing panic.
In the midst of all the speculation a salient point is being ignored: Ahminejad just announced he didn't need nukes.
Would a Persian have terrorist assets attack the running of a race commemorating a Greek victory over the Persians?
It’s possible about the battle, but I’m sticking with Occam’s Razor until something points another direction.