SK24, which reported that Henri Helminen was exiting his van when smoke began to emanate from his pants pockets. In the closed-circuit video, embedded below, Helminen can be seen removing the reportedly 90-day-old iPhone 4S from his pocket and tossing it to the ground, where it seems to flare like a smoke grenade.
According to SK24, Helminen chose to go to work early that morning, else he would have been sleeping next to the device when it ignited. It's unclear, though, whether or not that means that Helminen actually places the phone under his pillow (some do!) or just has it next to his bedside.
This sort of thing has happened before, and Li-ion batteries have been recalled before for such devices as laptops. The location of the incident - Finland - has raised some suspicions, though.
After all, Finland is the home of Nokia, maker of Lumia Windows Phones (and former maker of Symbian phones). 9to5 Mac went so far as to say, "This is a little too close to Nokia’s headquarters for us to believe off the bat." It's not clear, though, if the incident occurred right down the street from Nokia HQ.