Might have something to do with this recent notice from the FAA:

http://www.faa.gov/other_visit/aviat.../SAFO10017.pdf

Up until now, there have been restrictions on the big lithium-ion packs used for professional video cameras and the like. But after the UPS crash, they seem to be taking a hard look at all shapes and sizes. I'd never put liths in a checked bag--at least in a carry-on bag, you have a chance at extinguishing a fire.