What is surprising about the Boston bombing isn't that it happened, but rather that it didn't happen before. I was pretty sure after 9/11 we would get more of these bombings in the next few years, not only from Muslim fanatics but also from home-grown fanatics and copycats. Other nations got them - think of the 2005 London bombings and the 2004 Madrid train bombings. Of course we did get the 1996 bombing at the Centennial Olympic Park in Atalanta, which turned out to be from a home-grown anti-abortion fanatic. And the 1995 Oklahoma City Federal Building bombing, which also came from a group of home-grown militia fanatics.
But considering how open and free our nation is, how porous our borders are, and how long the middle-Eastern wars have dragged on, one really has to credit our domestic anti-terrorism forces with preventing the sort of frequent bombings that have plagued Iraq and Afghanistan for years now.
I suspect this will turn out once again to be the product of a home-grown fanatic of some sort. I do notice that American power has apparently gained some respect in the middle East, The Pakistani branch of Al Qaeda wasted no time in assuring everyone who would listen that it had ABSOLUTELY NOTHING do do with the Boston bombing. I guess that they noticed after 9/11 that when they took down two US buildings, we retaliated by taking down two whole countries. There is an Asian saying (actually used in one of Tom Clancy's novels): "Who pulls the tiger's tail needs to watch out for his teeth".