Besides my initial reaction to this, which was "oh shit, that's terrible, do I know anybody in Boston and if so are they alright?" my second response was to consider this question.

If no one admits to it or posts any sort of rationale, it ain't terrorism. Although it's been usurped for obvious reasons in the past decade, the term "terrorism" ought only be used in reference to violent acts perpetrated against person or property with the express intent of making a broader political statement against a particular governmental entity while at the same time undermining that entity's ability to govern effectively. Ultimately, what defines terrorism is not the target of the attack, but the target audience of the attack, and the express message pinned to the attack.

Such an attack can result in loss of life, but doesn't have to. And not every attack that results in widespread death or bodily harm necessarily counts as terrorism. Thus you can have acts, like those of the Earth Liberation Front in the late '90's, that don't hurt or kill anybody but are still considered terrorist acts; likewise, you can have something like last year's shootings in Aurora that kill several and injure several more, but won't be considered terrorism by anybody not hoping to distort the term for political purposes.

In regards to today's tragedy: until somebody owns up, the best we can say is that it's maybe incidental terrorism. That is to say, without any indicators pointing us towards explicit political statement, the immediate act ought to be considered nothing more than shitty-minded criminal mayhem. Secondarily, however, the act undermines our feeling of security within zones generally considered safe; it undermines our faith in the ability of the local government to keep its citizens safe. And the longer the perpetrator goes unidentified, it undermines our faith in the federal government's ability to successfully identify and prosecute the perpetrator or perpetrators. Hence, "incidental" rather than "explicit" terrorism: no immediate statement claiming political intent, but an as of yet unintended political reverberation.

I suspect that, after careful initial political triangulation, we'll hear intimations toward- if not an explicit spelling out of- terrorist intent on the governmental side of things. While it would clearly be in the Administration's best political interest to play it off as criminal activity, I suspect the federal government is going to want the investigative and prosecutorial tools afforded to terrorist activity. Ergo, if they can establish an early link to terrorist intent, they can avoid a lot of the investigative snares that tangle up criminal investigations, and then if and when they catch the perpetrator they can prosecute accordingly while avoiding potential claims that they at any point acted counter to due process and all that.

Above all of this navel-gazing, though: regardless of what else it was, today's incident was horrible. I can't figure out how anybody could do something like this. Imagining those in the crowd as it happened, I can't think of anything other than how fundamentally offensive to one's very state of being something like this must be. To go out expecting nothing more than a day of leisure with a group of friends or loved ones, and in a heartbeat have to be subjected to such confusion, horror and pain... goddamn. Hope everybody in this community, and theirs, are safe and emotionally okay.

EDIT: Nailed it