I lived in the USA, and I travelled there in 2006. It was very safe back then. If you're referring to the last 12-24 months, I am not so sure. I think it is safe due to tight security measures and fingerprinting etc.
For the most part it is safe, I think. There is always a risk, but to be honest, there is more chance that you will get hurt travelling to work day to day than there is of you getting caught up in a terrorist attack. You have to be one of the really unlucky ones for that to happen.