Three weeks ago I blogged an article by John Mueller entitled "Is There Still a Terrorist Threat?" It's definitely worth a close reading.
In short, Mueller argues that if al-Qaeda wanted to strike the United States in some fashion, it could have done so fairly easily by now. So the absence of an attack is an indication that there are no foreign terrorists in the United States, and that they aren't inclined (or perhaps aren't able) to make their way here to carry out attacks.
Anyway, I see that Foreign Affairs, which published Mueller's article, is sponsoring a roundtable discussion of Mueller's views.