The director of the University of Pittsburgh Cancer Institute warns his staff to limit cell phone use to minimize cancer risk. While no studies have detected a link, that does not necessarily prove that there isn't a link. It reminds me of the Bill James article "Underestimating the Fog" (PDF) in which he noted that just because past studies haven't detected clutch hitting doesn't mean it doesn't exist.
Cell phones and bluetooth headsets emit non-ionizing, microwave radiation. That much we know. Do they increase your risk for brain cancer? To conduct a study of that magnitude would take years and years and cost millions of dollars. It's unlikely anyone will fund a study like that.
So we're all part of a real world experiment. Here's how I see it playing out. Some people will get brain cancer 20 years from now from high cell phone use, and they will bring massive lawsuits against the cell phone companies. But one special person will gain superpowers from all that radiation, a sort of slow burn Bruce Banner. But this hero's powers will only be active in large metropolitan areas, will wane when going through tunnels or riding in elevators, and will come with some inexplicable state and local taxes.
One risk from bluetooth headsets that has been confirmed: wearing one will make you look like an idiot.