With the suicidal non-Muslim folks that keep popping up, I would expect deliberate attempts to be quite frequent...
But if they want to kill a lot of people, they can do it on the ground without having to use up a perfectly good suicide fanatic.
But there haven't been any major attacks against the US since Sept 11. I think the reason is that radical Islamists have realized that terror doesn't work on Americans the way it does on, for example, Spanish voters or Italians or the Clinton administration. When terrorists attacked us, they lost two host countries.