I came across this on another website, but here's the TSA announcement. I haven't really tried to figure out what is and isn't allowed, but since I get to fly on Friday I'm sure it'll be fun...
What they say is true - a shorted battery can cause a fire, and safety is most important. But I'm surprised they only talk about lithium batteries (hold a piece of wire across both terminals of a D cell alkaline and you'll get a surprising amount of heat - no, don't try this at home, and anyone hear urban legend stories of wrenches laying across terminals of a car battery when it was started {hint, "kaboom"}?). I guess my main gripe is enforcement - rules always seem to be enforced differently, and what do you do if you're at the airport with an expensive battery they decide you can't take with? Guess I'm just pro-road trip.