Mobile devices that utilize Lithium-ion or Lithium-polymer batteries are always at risk of catching fire or even exploding if they are overcharged, or if their battery pack gets damaged. Usually the culprit is the result of using a cheap charger that was not approved by the manufacturer.
Unfortunately for Apple, neither were the case in a recent incident involving the company's new iPad Air. The tablet in question was sitting idle on a display stand at a Vodafone store in Australia when it suddenly exploded and burst into flames. No one was injured in the incident, but firefighters had to be called to extinguish the flames and extract the smoke from the store. In true form Apple is staying silent on the matter and had sent a team to collect the iPad Air for analysis.
While this very well may be a one-off incident, I would not leave an iPad Air laying on something like a couch, bed or pillow until Apple releases a statement on the matter. After personally witnessing an iPhone 4 catch fire while on an official apple vehicle charger, I know how quickly the devices can go from functioning fine, to going up in smoke.