Research has for the most part failed to validate the cell phone-brain tumor hypothesis, but the information gaps that remain cause some neurosurgeons to be concerned enough to always use an earpiece when talking on their cells.
The largest gaps in our understanding relate to long term use and extensive use by young people, whose skulls are thinner than adults and who may eventually use their cell phones for many, many years. The devices have not been in widespread use for long enough to know what the long-term effects may be.
Update 6.8.08
A massive study has been approved in Britain to evaluate whether the long-term use of mobile phones causes brain cancer, and Alzheimer’s and Parkinson’s diseases.