SecurityMonkey has a post regarding new TSA guidelines, along with a video/link demonstrating how a small explosive may be created and hidden that is probably pretty darn undetectable.
He also hits on one of the major things I’ve felt since 9/11: reinforce the damn cockpit doors. *What makes plane bombings so different in scope to bus, subway, or even boat bombings is the ability to take control of the vehicle to do even more damage. Therefore, protect what you can. Safety on a public plane will never be assured, although you can help minimize some obvious things like guns or stupid terrorists.
* This also does include policy and thought on what can be passed to and from the cockpit, especially on long flights on emergencies, either mechanical or medical in nature (pee breaks?), and so on. Basically, you also don’t want pilots to be coerced into opening the door, or have something slipped to them (a sedative through a food tray?) that jeopardizes the operation of the plane because no one else can get in. (Then again, if you drug the pilots, the plane is probably doomed anyway…)