Batteries in checked luggage can cause fires.
Passengers on all planes, including cheap international flights, departing from the US will no longer be able to pack loose lithium batteries in checked luggage beginning January 1, 2008. The new regulation, implemented by the federal Department of Transportation is designed to reduce the risk of lithium battery fires.
The new regulations allow lithium batteries in checked baggage if they are installed in electronic devices (eg. you can pack your laptop with the battery installed in your checked luggage). Consumer electronics such as travel cameras, cell phones, and most laptop computers are still allowed in both carry-on and checked luggage. Additionally, any number of some types of lithium batteries, such as the ones used in cell phones and laptops, can be taken in carry-on luggage if properly stored.
Up to two of the more powerful lithium batteries used in professional audio/video/camera equipment may be taken in carry-on luggage according to the new regulations.
Consult the Department of Transportation, SafeTravel web site for more information on how to pack for air travel.