If you’re a photographer working in (or passing through) the US, you may want to take a look at this post on Chase Jarvis’ blog to find out more about a new regulation that has just gone into effect in the United States regarding the transportation of Lithium-ion batteries (the kind that may power cameras, flashes, laptops, etc.).
Long story short, there are now limits to how many spares you can carry and you will likely have to carry them in your carry-on baggage. The rationale behind this change is that if the batteries catch fire, they can potentially burn hot enough that the fire extinguishers in the baggage compartment may be insufficient to put out the fire and there are better systems in place in the cabin to handle such occurrences.
Flyingwithfish.com has some more information here and also has an interesting suggestion on how to carry more spares here: if the battery is installed it is not considered a spare. If the battery is in a charger, it is considered ‘installed.’ This may or may not get you through security with an extra battery or two, but it’s an interesting thought.