There’s an article with a quote from an airline person saying: "They're no more hazardous than any other battery-powered piece of equipment," Hill told eWEEK in an exclusive interview. Hill said that while such batteries can catch on fire, "If you're carrying it into the passenger cabin, the flight attendants should be capable of using an extinguisher and controlling it easily."

However, I don't think the cabin fire extinguishers could put out a LiION fire. There have been some FAA tests that say the cargo cabins (which have a halon system) would be effective as long as the pressure from a battery explosion didn't blow a hole in the pressure containment of the cargo cabin.
And if you can trust an engineer more than some airline official, they say

And an article here: "Here's the short version: if you abuse a lithium battery, it can experience a "discharge with flame" and you can't put it out. It generates its own oxygen to keep the fire going. Also, as we discovered this week, errors in manufacturing can give them the potential to self-ignite."