The US Postal Service will prohibit the shipping of lithium ion batteries to international destinations from 16 May, 2012, because they are considered a fire hazard.

The prohibition affects all batteries containing lith ­ium metal or lithium alloy. It applies to consumer-type primary (non-rechargeable) lithium cells or batteries and secondary 
rechargeable) batteries used to power flashlights, cameras, portable computers and cell phones and affects batteries that are pre-installed as well as those packed with the equipment but not installed. Individual batteries are also included in the prohibition as well as damaged or recalled batteries.  Batteries can still be mailed to addresses within the US and its territories under certain conditions.

Full details can be found at