Effective May 16, 2012, the Postal Service will revise Publication 52, Hazardous, Restricted, and Perishable Mail, to indicate that primary lithium metal or lithium alloy (nonrechargeable) cells and batteries, or secondary lithium-ion cells or batteries (rechargeable), are prohibited when mailed internationally or to and from an APO, FPO, or DPO location. However, this prohibition does not apply to lithium batteries authorized under 349.22 when mailed within the United States or its territories.
International standards have recently been the subject of discussion by the International Civil Aviation Organization (ICAO) and the Universal Postal Union (UPU), and the Postal Service anticipates that on January 1, 2013, customers will be able to mail specific quantities of lithium batteries internationally (including to and from an APO, FPO, or DPO location) when the batteries are properly installed in the personal electronic devices they are intended to operate. Until such time that a less restrictive policy can be implemented consistent with international standards, and in accordance with UPU Convention, lithium batteries are not permitted in international mail. The UPU Convention and regulations are consistent with the ICAO Technical Instructions for the Safe Transport of Dangerous Goods by Air (Technical Instructions). The Technical Instructions concerning the Transport of Dangerous Goods by Post do not permit “dangerous goods” as defined by the ICAO Technical Instructions in international mail. Currently, the only exceptions to this general prohibition relate to certain medical materials, infectious substances, and radioactive materials when they are treated in accordance with additional requirements listed in the Technical Instructions. Lithium metal or lithium alloy batteries and lithium-ion cells are listed in the Technical Instructions as Class 9 Miscellaneous Dangerous Goods. The prohibition on mailing lithium batteries and cells internationally also applies to mail sent by commercial air transportation to and from an APO, FPO, or DPO location.
So it's both primary and secondary cells which are banned – and the decision was forced on the USPS by the UPU and ICAO (the latter, presumably, because of the recent incident where a cargo plane fell out of the sky after a crate of lithium ion batteries caught fire at 35,000ft and couldn't be extinguished.)
"If it ain't broke, don't fix it." - Bert Lantz