According to the Federal Aviation Administration (FAA) e-cigarettes can be considered a fire hazard if placed in checked baggage.
The FAA released a Safety Alert for Operators (SAFO) directed towards U.S. air carriers from the International Civil Aviation Organization (ICAO) who gave several incidents caused by e-cigarettes. ICAO's recommends banning them in checked baggage.
Incidents that have occurred are overheating or catching fire when the heating element was accidentally activated. Last August an e-cigarette in checked baggage caused a fire and forced evacuation of the airplane. When users make their own parts for the e-cigarettes that could cause more danger.
The FAA SAFO encourages airlines to communicate this new rule to passengers in the future.