From January 1st the US Transportation Security Administration (TSA) has issued new regulations with regards to carrying Spare Lithium batteries.
Effective January 1, 2008, spare lithium batteries - extra batteries not installed on devices - will no longer be allowed in checked baggage. Spare lithium batteries may be packed in carry-on baggage and lithium batteries installed in a device may be packed in either checked or carry-on, as long as the battery is installed in the device.
With all the furore over the laptop battery recalls I'm not surprised by this move.
You can get more details on the change in regulations HERE