PT,
I noticed that too. I've read both 'lower temp usability' as well as keeping them above freezing. The nice thing is though, you can keep them in a heated environment without having to ventilate/exhaust them, and therefore cut way down on excessive heat loss.
I recently bought a set of motorcycle hand grip warmers to use for battery warming. I plan to envelop them in a non-flammable material, like a welding blanket material, in case of a short. The material will also help cushion them seeing as the batteries will be sitting on top of them.