I've seen it posted twice in the forums so I'm putting it here.
The US has put a "ban" or limitation on carrying on and packing spareLithium batteries on airlines.
http://safetravel.dot.gov/whats_new_batteries.html

This is for SPARE LITHIUM batteries, not NiMH, Alkaline, Lead-Acid, or any other type.

If you don't know the types you shouldn't be carrying batteries.

But as a very frequent flier this would greatly affect me if it was all spare batteries.

Every time you don't want your battery to die and be screwed without an extra, the terrorists win.

funny how the reports on these batteries exploding have been installed in a device, e.g. laptops, phones. I dont think i ever saw one of a spare battery exploding.

WTF? That is so ******ed. It is a frickin' spare battery. They don't just combust whenever they feel like it.

This is for CHECKED baggage only. You are allowed to carry spare batteries in your carry-on bags. Very clearly stated.

R.
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The news articles say that there was an actual incident in which firefighting teams couldn't put a blaze out because it was fueled by exploding batteries.

There is a limit on how many grams or something you can carry on. How do I know how many whatevers my batteries have? Does it say on the battery?

That includes N800 batteries. What is not clear to me is what is the limit on the actual number of ‘up to 8gm equivalent’ batteries you can carry.

The phrasing of the rules is ambiguous, the GUIDE TO RULES EFFECTIVE JANUARY 1, 2008 states “Spare Lithium-Ion Battery” i.e. singular

I take 5 spare N800 batteries with me on long journeys. Do I need to aggregate the “equivalent lithium content" of each battery and stop if 8gm is reached? Or am I only permitted to take 1?

The bozo at the check-in certainly won't know and will make an arbitrary decision to confiscate all of your batteries bar one just to be safe and keep their job.

Also don't forget the packing requirements, tape over the terminals and each battery to be in its own plastic bag.

The facts can be found here.

For camera, PDA and cell phone batteries, and most laptop batteries, the only restriction is that spare batteries (not installed in a device) must be in your carry-on.

Details: Lithium-ion rechargeable batteries installed in devices - permitted, up to 300 watt-hours.

Spare lithium-ion rechargeable batteries up to 300 watt-hours: Must be in carry-on baggage. Batteries over 100 watt-hours limited to 2.

All camera, PDA and cell phone batteries, and most laptop batteries, are under 100 watt-hours, so you can take any number of spares but they must be in your carry-on.

The BP-4L battery in the N810 is a big 5.55 watt-hours. It is a lithium polymer battery, which is a type of lithium-ion battery, not a lithium metal battery.

To compute the watt-hours of a battery, multiply the voltage by the mA-hours and divide by 1000.

There are more stringent restrictions on lithium metal batteries (coin batteries, and lithium AA batteries, for example). Spares of these would have to be carry-on, up to 25 watt hours.

Lithium metal batteries are a bit more dangerous, as the lithium can (will) burn if it gets in touch with humidity (even air humidity). For the more common lithium-ion batteries the main concern is that it's very easy to short the contacts (as they are usually on the same side of the battery) if the spare battery is loose in your luggage. The short can create a fire. This is much worse if it happens in the storage bay than if it happens in your pocket inside the cabin.

The press has completely blown this out of proportion because they didn't do their homework before announcing it.

This applies mainly to Lithium Metal batteries (the non-rechargeable type, such as CR123) and not to the more common Lithium Ion or Lithium Polymer batteries commonly found in cameras, cell phones, and laptops. As mentioned before, it also only applies to checked baggage. Lithium Metal batteries are considered dangerous because the lithium is not chemically bonded to another material, so if the battery were cut open raw lithium would become exposed to air and react violently. Lithium Ion and Lithium Polymer batteries can be cut open without having this reaction (although I still wouldn't recommend it). Explosions due to shorting are only of secondary concern.

Unfortunately, most people are clueless to the difference and take the press at their word. Just wanted to set the record straight, for all the difference it will make.