I can't remember who had it... maybe it was Discovery Channel's special on dragons, but there was the theory that dragons spew some sort of liquid chemical that could combust at particular temperatures, or that they could belch some gas with a low ignition point. There was some biological science to the possibility of breathing fire or something, but I think they may have come to the conclusion that it would have been more of a deterrent than an actual threat, kinda like flaps of skin that expand when stressed.