In an effort to protect the public from lithium batteries (and aren’t we all afraid of them) the DOT enacted new lithium battery rules for travel beginning on January 1st of 2008. Basically the rules state that you cannot pack spare lithium batteries in your checked baggage. Meaning the bags you hand over to the clerk at the counter. And when they say spare batteries they mean loose ones floating around. If you have lithium batteries in your camera, MP3 player or other device those are just fine.
If you need to carry spare lithium batteries then you’ll have to pack them into your carry on baggage. Generally you can carry any number of them with you onto the plane. If you have any questions about what batteries and how many you can bring, check out the official DOT page to get more specific information.
The stated reason for this lithium battery rule is that lithium batteries are a potential fire hazard and when left in checked bags they’re apt to spontaneously combust. By bringing them onto the plane with you the government feels that the in-flight crew is better able to closely manage the situation should a battery burst into flames. Although anyone with some sense of logic would understand that the checked electronics items with batteries in them are probably just as likely, or more likely, to burst into flames as the ones you’ve put in your carry on.