Transportation Security Administration - http://www.tsa.gov/   
(for the latest updates on security, etc.)

Update: Jan. 1, 2008 :  New D epartment of Transportation Rule for Passengers Traveling with Lithium Batteries

The new rule on lithium batteries in air travel, which took effect on January 1, 2008, prohibits carriage of spare lithium batteries in checked baggage, such as large suitcases you hand over to the airline for handling. If you put a portable electronic device in checked baggage, you may still do so with the batteries installed in the device.  

You may still carry lithium batteries in your carry-on baggage, provided you follow some guidelines.  For details on carrying lithium batteries in your carry-on baggage, please go to: http://Safetravel.dot.gov or read more...