The rather credible-sounding reports that I've heard have not been that cell phones "start fires" per se, but that when people try to save money by buying cheap, off-brand replacement batteries for their phones that these batteries have, on occasion, actually exploded. There were numerous reports of this happening and documentations of the sometimes serious injuries that have resulted.
None have been reported, to my knowledge, in cases of people using high-quality batteries.
You wouldn't happen to have a link to the article you refer to, by any chance?
Nope, Mythbusters is a cable show.