The TSA has discovered that on January 1st, 2008 lithium batteries are going to become more dangerous than they were on December 31, 2007. Thankfully, they've taken action by forbidding them beginning in 2008.
Lithium Metal Battery, Spare or Installed (over 2 grams lithium):
In checked bag? Forbidden
The Department of Transportation's web site doesn't say why they're forbidden. They just are.