You better check these NEW rules for batteries!
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
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.
Read the NEW rules here at Safe Travel
Special thanks to Rob Bushway from GottaBeMobile for this tip!