I have been following news on the health effects of radio frequency radiation for twenty-five years, and I remain concerned about the possible effects of being bathed in microwave frequency radiation from cellphones, portable phones, and wireless Internet adapters. Keep in mind that all those devices use the same frequencies that a microwave oven uses to turn hot dogs into charcoal, albeit at lower energy levels. There has been a long, muted debate about the dangers, with the industry steadfastly maintaining there is no health risk, and some scientists and researchers much more concerned. Keep in mind that the tobacco industry maintained for decades that there was no health risk from smoking.
This study from the UK suggests that mobile phones may be a source of brain tumors. The study seems broad enough in scope to produce good data, so I view it as an encouraging sign. The researchers interviewed thousands of people with brain tumors to find out about their cellphone use, and were not able to find a connection between use the incidence of tumors. I don't think the debate is over, though. One possible flaw in the study--getting a brain tumor often means you may die in twelve to eighteen months, and the researchers admit that the data could be skewed because there were unable to interview people that had already died.
Until this is all sorted out, I would recommend using a wired headset or earpiece as much as possible with your cellphone. Remember that those wireless Bluetooth earpieces also use microwave frequencies to communicate with the phone.
Update: This study was funded by several cellphone manufacturers, which makes the results more suspect.