Doing some checking .... if the bad guys figured a way to extract the lithium metal from Li-Ion batteries then they COULD blow up planes. Lithium reacts very violently with water.
I read somewhere just recently that a UPS plane carrying a large shipment of lithium batteries caught fire on the ground. UPS has stated they will not ship these items any more. Like the ones Dell has in their recalled notebook computers.
If they can catch fire for no reason who's to stop terrorists from using them somehow to set the plane on fire or figure out how to use them in an explosion of some sort.