The government has issued new rules for air travel with spare lithium batteries. These rules are effective January 1, 2008:

The following is a direct copy and paste from the SafeTravel.dot.gov site, which you can read for more details

• 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.

The following quantity limits apply to both your spare and installed batteries. The limits are expressed in grams of “equivalent lithium content.” 8 grams of equivalent lithium content is approximately 100 watt-hours. 25 grams is approximately 300 watt-hours:

• Under the new rules, you can bring batteries with up to 8-gram equivalent lithium content. All lithium ion batteries in cell phones are below 8 gram equivalent lithium content. Nearly all laptop computers also are below this quantity threshold.
• You can also bring up to two spare batteries with an aggregate equivalent lithium content of up to 25 grams, in addition to any batteries that fall below the 8-gram threshold. Examples of two types of lithium ion batteries with equivalent lithium content over 8 grams but below 25 are shown below.
• For a lithium metal battery, whether installed in a device or carried as a spare, the limit on lithium content is 2 grams of lithium metal per battery.
• Almost all consumer-type lithium metal batteries are below 2 grams of lithium metal. But if you are unsure, contact the manufacturer!