From http://www.interesting-people.org/archives/interesting-people/200608/msg00087.html, via http://www.schneier.com/blog/:
So, we've covered in the lab and in the bathtub. On an airplane? On an
airplane, the whole thing is ridiculous. You have nothing to cool the
mixture with. You have nothing to control your mixing with. You can't
take a day doing the work, either. You are probably locked in the
tiny, shaking bathroom with very limited ventilation, and that isn't
going to bode well for you living long enough to get your explosives
manufactured. In short, it sounds, well, not like a very good idea.
And now, on to the fun part of this note. First they came for the nail
clippers, but I did not complain for I do not cut my finger nails. Now
they've come for the shampoo bottles, but I did not complain for I do
not wash my hair. What's next? What will finally stop people in their
tracks and make them realize this is all theater and utterly
ridiculous? Lets cut the morons off at the pass, and discuss all the
other common things you can destroy your favorite aircraft with. Bruce
Schneier makes fun of such exercises as "movie plots", and with good
reason. Hollywood, here I come!
And he goes on to describe some possible attack methods.