The FAA has recently passed a new rule that limits what you can do with lithium batteries while traveling on airplanes in the US. This only applies to Lithium batteries, not to other types of batteries like NICad or NiMH.
Moderator Cor Bosman has summarized the effect this new rule will have on divers and underwater photographers. “Almost all batteries and devices that we as divers and photographers carry will not be impacted by this rule. This means you can still travel with them, but you must carry any spare batteries in carry on. “
He has also posted a formula you can use to determine the lithium content of your batteries.