Lithium battery fires, while rare, do occur on aircraft. The FAA recorded 138 incidents between March 20, 1991 and October 9, 2012 at this link. Most of these are related to laptop computers and batteries in cargo compartments. The length of this list provides some indication of why the FAA is so concerned about issues with Lithium-ion batteries.
Our friends at Avitas even have a free course on what to do in case of a Lithium-ion battery fire on board. This course takes only about 10 minutes, but is highly recommended for anyone who has a laptop or cell phone in which a problem could potentially develop, especially if you drop the device. Take the course at this link. Even if you are not a flight crew member, and only a laptop or cell phone user, you should understand why these batteries catch fire, why water is the best way to put them out, and why you should never use ice! Well worth the few minutes it takes just in case it happens to you.