Just like I've been saying for years - the security at airports is to make us feel secure so we keep flying and the airline industry stays healthy. Whenever they pull anything out of my bag and deem it a security list I have to keep mum about the fact that I'd already gone through security at 7 airports previous to this one and they just found the offending item. I'm certain that anyone with a brain could carry enough parts on a plane and blow a hole in the side. The reality is there just isn't enough people wanting to do it. Terrorism isn't about keeping innocent people from doing things, it's about scaring them.
According to a USA TODAY article the TSA (Transportation Security Administration) had agents in plain clothes carry phony bomb parts through security at LAX and Chicago O'Hare to see which would get spotted. Seventy-five percent of the parts taken into LAX got through without detection, and 60% at O'Hare. SFO (San Francisco/Oakland) hires private companies to do the screening and only 20% of the parts went undetected.
A test in 2002 showed they failed to find guns 24% of the time. In the late 90's they missed 40% of bombs, by 2002 they were missing 60%. In the next 5 years they might as well dissolve the TSA altogether for ineffectiveness.
This basically means that if there's a terrorist on the plane you should probably go ahead and violate the FAA's regulations and call your loved ones on your cell phone in hopes that it will mess up the aircrafts navigation system (It won't).
It seems their ability to detect completely assembled bombs has gone up though (as long as the word BOMB has been stenciled on the side in block letters using lead paint).