Stars and Stripes has a very concise article by David H. Schanzer answering that question. The fact is that despite the recent Boston attack, the US has very few terrorist attacks, especially given that we are wide open, free wheeling society compared to most of the world. Go read.
While the horrific bombing at the Boston Marathon has brought concerns about terrorism back to the forefront of national attention, it is worth remembering that terrorism inside the United States is exceedingly rare. Over the past 40 years, about 11 people per year have been killed by acts of terrorism (excluding the 9/11 attacks). To put this in some context, over 122,000 Americans died from accidental injury in 2011, while 53,000 died from the flu and pneumonia. Terrorism is also rarer now than in past decades — there were 1,357 terrorist attacks in the United States in the 1970s, but only 168 in the 10 years after 9/11.