This much is clear: cell phones do something to your brain. The exact effect is still up for debate.
A new NIH study indicates that holding a phone close to your head causes an increase in glucose metabolism. That's associated with brain activity, which under normal circumstances is a normal process. You get the same effect if you simply concentrate real hard, according to the Gate.
What's still unknown is whether there's any long-term effect of stimulating activity though external means. It could be as harmless as intense thinking, or it could have more long-lasting impacts. Either way, the discovery that phones have a measurable impact on brain activity is significant.
In fact, we probably won't know for sure what the long-term impacts are until enough time has passed to study them. That means you'll need to wait a decade or two for conclusive research.
Until then, if you consider your brain an object worth protecting, you can't go wrong with a headset. Of course, keeping your phone in a pants pocket could have unexpected effects on your thigh.
Cell phone companies, not surprisingly, are quick to point out that most research indicates that there are no physical consequences to using cell phones. Of course, much of that research has been funded by the cell phone industry.