I don’t understand why it matters whether President Obama is referring to the Boston bombings as an “act of terrorism” or not.
When explosions happen, and it’s determined that they were the result of multiple bombs detonating in concert, it’s “terrorism” by default. There’s no other description for it.
Everything else seems like a straightforward, albeit difficult, process: You find out who did it, you find out why they did it, and then you make your judgment and mete out the appropriate punishment.
Until then, what’s the point of arguing over semantics?