There are three suspects in my mind at the moment:
1) Islamic militant - Probably not a full Al'Qaeda member but a wannabe or from one of the African zones.
2) Political activist - Ala Breivik or McVeigh, could be either side of the political spectrum
3) North Korea - The most unlikely of the three, but given recent events and todays date means that a few small warning lights are flashing.
In regards to choice of target, at first glance it doesn't seem to make sense for an Islamic militant, however given that the big names like New York and DC are pretty well secured, and this event was an international one, it's possible, it is possible.
If it isn't, and it is a political nutjob, then God help America because it's going to divide an already quite well divided nation, and if it's a person with a pro-2A agenda then the rhetoric is going to get even nastier than it is now, and it'll only be a matter of time before someone is copy and pasting conspiracy theories that Obama set this up in order to push through his firearm control law (especially when you consider that Biden was in a meeting with supporters of such a law when he got the news) on GT.
However, this is all matters to be concerned about tomorrow, what is important right now is that the casualty rate remains low, and that any other devices in the area are found and deactivated.
If anything, the amount of EMT people at the marathon and within easy reach of the marathon will help here.
Here's a video making the rounds of the explosion and immediate aftermath, I didn't spot any gore on it, but obviously be aware:
http://www.boston.com/video/viral_pa...=2303076923001