I think the no fly list is a pretty big deterrent... its probably hard to devote time, and resources, to a plan if there's no guarantee your guys ever make it onto the plane. Having the same last name can get you pulled, so even if they have "clean" guys, they could get pinched.
Also the public and acquaintences are all more aware now, so its harder to not get pinched ahead of time - see the case of this kid here in OR, who i even spent some time with... i believe the deal is that his father notified the fbi, because his son was exhibiting some extremeist behavoir... he didnt have a plan, or anything, but just his general demeanor was enough to raise red flags, with his own family.
This certainly isnt everything, but i do believe that keeping would-be terrorists out of airports, out of schools, and ultimately out of the country should be the priority.