As of May 16, 2012, USPS has changed part of their mailing rules and standards. Lithium batteries and cells (both rechargeable and non-rechargeable) are now not allowed to be shipped internationally or to any APO, FPO,  and DPO locations. This means that mailers also cannot send out products that have lithium batteries inside of them, including tech gadgets such as cell phones, MP3 players, laptops, and digital camera.

After January 1, 2013, limited quantities may be allowed to ship out internationally, as long as "the batteries are properly installed in the personal electronic devices they are intended to operate."

This does not affect the current rules for mailing lithium products within the United States, and lithium batteries (and their corresponding technical products) may still be shipped out normally. However, for international shipping, mailers may have to resort to private carriers such as FedEx.

Source: USPS