The Vallejo fire department recently determined that a fire must have started in the victim's pocket; since the only item they found in the pocket was a cell phone, they announced that the cell phone must have started the fire. After this announcement, Nokia engineers flew in to perform some tests:

Tweedy said the Fire Department called Nokia in because "they offered their electrical engineers to come in and do testing that we can't afford to do."

According to the article, after the investigators checked the wiring, the fancy-shmancy super-expensive conclusive test was pressing the power button to turn the phone on.

ETA: While researching the first comment on this post, I found a blog entry about this subject with perhaps the perfect title: "Nokia isn't to blame for spontaneous combustion in man's pants".