I heard that you are not allowed to bring any extra li-ion batteries in your carry on anymore. The batteries must be installed in the device. I hope you didn't find this out the hard way!
According to the FAA, most laptop lithium-ion batteries ARE allowed in carry-on baggage, and in addition to the battery in the laptop, passengers can carry on up to two extended-life li-ion spares. See here for the notice.
Interestingly, spare lithium-ion batteries are NOT allowed in CHECKED luggage.
I heard that you are not allowed to bring any extra li-ion batteries in your carry on anymore. The batteries must be installed in the device. I hope you didn't find this out the hard way!