None, but that doesn't mean TSA is effective. The fact that so many people have accidentally breached security should indicate to anyone with a rational mind that a someone with intent to circumvent security would be able to do so.
The fact that there have not been any attempts doesn't count in TSA's favor. If you start working backwards from 9/11, the next hijacking you find of a US airliner occurred in 1985, and that was a flight from Rome to Athens - still outside TSA's reach. If you go back to the next flight that was a US-originating hijacking you are back to 1980, 21 years before 9/11/01. So there is a history here of long periods of time between attempted hijackings.
Combine this with intelligence that Al Qaeda is focusing on other targets and the TSA becomes irrelevant in the security landscape. The real reason I think the focus is off airplanes is that they have taken hijackings as far as they can. By turning aircraft into weapons they upped the stakes, back in the 60s, when hijackings were at their highest rate in history, passengers were content to sit quietly and let negotiators and hostage rescue teams take care of the problem because that was their best chance for survival. Now the world knows that their best chance to survive a hijacking is to rise up against the hijackers. If the hijackers can't gain control of the plane then the whole thing is pointless.
TSA has consistently sent the message that they are there to catch or discourage potential threats, but these people are ready to die, they want to die. They don't care if they get caught, if you corner a terrorist with a bomb he will just detonate it there rather than at his intended target. Anything less than a bomb isn't going to be enough to take over a plane, a knife, a gun, it won't matter because the passengers simply won't allow it to happen anymore. You can't get into the flight deck to take control, and even if you did, some flight crews now carry firearms and they have a really nasty axe up there.
I've said it before and I'll say it again, TSA's entire rationale for its existence is the same as me building a giant microwave ray in my back yard that repels comets and then claiming that it works because no comets have struck the planet.