As of May 2011, The World Health Organization has listed cell phone use in the same carcinogenic hazard category as lead, engine exhaust, and chloroform.
Most cell phones come with a wired ear piece and that is so you keep the phone away from your body. If you read the fine print that is in your phone manual, most will tell you to NOT keep the phone next to your head, or even in your pocket. Apple iPhone will tell you to keep the phone 5/8 inches away from body when transmitting. BlackBerry Bold says to keep it 0.98 inches away from your body when device is in use. Even when just keeping the cell phone in your pocket or next to your body, the manufacturers cannot guarantee that the amount of radiation that you will absorb will be considered a safe level. This does not sound very encouraging.
Using a wired headset will decrease the amount of radiation exposure because the phone is away from your body. Every inch you can get away from your body is less radiation that you are absorbing. You can even buy a ferrite bead for a few dollars at most electronics stores. This attaches to the wire and will absorb radiation traveling through it, reducing the amount that enters your body.
Speakerphones can be annoying but experts say that using this function is helpful because it keeps the phone away from your brain. Holding your phone just two inches away from your body decreases the radiation by a factor of 4.
Bluetooth earpieces will still expose you to some radiation but it is much less that a cell phone. Switch it from ear to ear so you don't have too much exposure on just one side.
Your cell phone will emit the highest radiation when connecting to cellular towers - such as when you are moving, like driving - where it connects from tower to tower. This will automatically increase the power to a maximum as the phone repeatedly connects to a new antenna. Weak signals also cause your phone to work harder thus more radiation. Your cell phone gives off more radiation when transmitting than receiving. Also, when your battery is low - there is more radiation output as well.
Text messaging is safer due to the phone not being closely held near your head of body - even though it is still in your hands.