I was downloading some rate info from the U.S. Postal Service website and started snooping around and found this in a May 2012 publication.
Here is a link to the .pdf publication on the USPS website.
http://about.usps.com/postal-bulletin/2012/pb22336/pdf/pb22336.pdf
Information on the lithium batteries starts on page 21.
DMM Revision: Mailings of Lithium Batteries
Effective May 16, 2012, the Postal Service™ will revise
Mailing Standards of the United States Postal Service,
Domestic Mail Manual (DMM®) section 601.10.20 to codify
that primary lithium metal or lithium alloy (nonrechargeable)
cells and batteries or secondary lithium-ion cells and 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 DMM 601.10.20 when mailed within
the United States or its territories.
Here is a link to the .pdf publication on the USPS website.
http://about.usps.com/postal-bulletin/2012/pb22336/pdf/pb22336.pdf
Information on the lithium batteries starts on page 21.