Statistics game (Score 3, Informative)

There's two issues here: There's the issue of whether current (or more stringent) security measures can still be beaten by a determined foe, then there's the issue of actual Li-Ion batteries going kaboom. I'll address the later.

Li-Ion batteries are some of the highest energy-density storage devices available to the general public. As such, they *are* dangerous. I design battery packs for a living, and let me tell you - if not for microprocessors and safety circuits, we wouldn't use Li-Ion batteries.

They are the only batteries that I know of that can fail dangerously when over-discharged. You start creating internal shorts of lithium whiskers between the cathode and anode, which bypasses any cell safety circuits.
They go boom very spectacularly if you overcharge them. The model RC heli folks have found this out the hard way, as they tend to run bare cells without protection circuits to save weight. (http://www.youtube.com/watch?v=hcwOwf55Rtc).
They have very low internal resistance, which means in a short circuit, they can release energy very quickly. Every manufacturing engineer at the company I work at have welded calipers to cell tabs, from accidentally touching the wrong stuff while taking measurements.

For a good cell manufacturer - and I'm talking about the LiShens, Sanyos, Kokams, and Panasonics of the world, the failure rate is 1 in 1 million. It's just a fact of life. The fly-by-night operations in China, responsible for some of the god-awful counterfeit cells out there, god knows what those failure rates are. And the vendors who use these cells tends to not put in the safety features (look up a BQ20Z70 chip, for example) to make a failure more likely.

The nightmare scenario would be some dude getting some last minute work in at the terminal, plugging the battery in for charging. Then the plane takes off with the laptop in the overhead compartment where the oxygen lines for the safety masks are kept, and the cells let go. Judging from how much energy a single 18650 cell can contain, it could easily do some very serious damage.

With the prices on Li-Ion dropping and more devices using them, it's no wonder that almost all of the 22 incidents reported occured in the last 3 years. Still a small number considering the amount of airplanes in the air at any given time, but enough for someone to pause and think...