Wagner James Au says:
"Andrew Sullivan has an interesting take on [this], derived from Ron Suskind's recent book One-Percent Doctrine: Deep Inside America's Pursuit of Its Enemies Since 9/11, (which is, it's worth pointing out, generally and scathingly critical of the Bush administration):
Why liquids? What were these weapons? One possibility is hydrogen cyanide. Ron Suskind's book revealed the terrorist breakthrough in a device called a "mubtakkar" that can be easily concealed in a carry-on bag and once detonated, kills everyone in a confined space within minutes. It's a variant of the Zyklon B innovated by the Nazis.
"Takeaway: just because TSA staff are incompetently dumping passenger liquids in public doesn't mean there's not a legit concern here."
From the Wikipedia entry on Mubtakkar:
The mubtakkar is described as a small binary chemical device that would generate large amounts of hydrogen cyanide gas, which could potentially kill hundreds in an enclosed space. The components contained in two separate containers would not be lethal to humans if individually released, so these bombs can be assembled, stored, and transported without appreciable danger. However, when the device is put into operation it releases large quantities of a lethal gas.
So, it appears that dumping liquids together into the same bin in a crowded airport is just about the worst thing the TSA could be doing right now. Why not leave them in their containers?
