New rules from TSA regarding lithium batteries begin on Jan 01, 2008

http://safetravel.dot.gov/whats_new_batteries.html

Effective January 1, 2008, the following rules apply to the spare lithium batteries you carry with you in case the battery in a device runs low:

• Spare batteries are the batteries you carry separately from the devices they power. When batteries are installed in a device, they are not considered spare batteries.
• You may not pack a spare lithium battery in your checked baggage
• You may bring spare lithium batteries with you in carry-on baggage – see our spare battery tips and how-to sections to find out how to pack spare batteries safely!
• Even though we recommend carrying your devices with you in carry-on baggage as well, if you must bring one in checked baggage, you may check it with the batteries installed.