There have been some studies showing an increased risk of malignant brain tumors with long term use of cellular and cordless phones. The long term risk seems to be over 25 years but it will probably depend on how much time the phone is close to your brain.
Of course we will not give these devices up but the goal would be to keep the phone as far away from your brain as possible by using the speaker mode or an ear bud with the phone somewhere else or Bluetooth in the newer vehicles. Also, the trend towards texting would actually be better since the phone is away from your brain.
Be especially cautious of your kids because the brains of children may be more sensitive and of course they are starting earlier than adults so as they get older the increase in these malignant brain tumors may skyrocket.
The risk is not large but seems be there so just make the appropriate adjustments.