Effective January 1, 2008, the US Department of Transportation has imposed new regulations on lithium ion batteries for air passengers in both carry-on and checked baggage.
The most important change is that spare lithium batteries, not installed in a device and over a certain size, may not go into checked baggage. Also, the number of spare batteries carried on board is limited by the total lithium equivalent in grams.
A complete description of the new regulations along with a chart summarizing the rules is at the safetravel section of the the dot.gov Website. With so many people descending on Macworld very soon with their computers and cameras, this might be a good time to review the new DOT regulations.