This is from an article in Aug/Sep 2008 issue of Scientific American Mind by R. Douglas Fields:
It appears that cell phone use prior to bedtime can contribute to insomnia. Cell phone use increases alpha brain waves. Alpha waves typically are high when the mind is trying to shut out the external world and create internal thoughts. Neuroscientist Rodney Croft and his colleagues at Swinburne University of Technology in Australia believe that the brain is boosting alpha waves to overcome the electrical interference caused by pulsed microwave radiation from cell phones that cause disruptions in brain circuits.
Another study by sleep researchers at Loughborough University in England people exposed to cell phones for 30 minutes in talk mode took twice as long to fall asleep as people exposed to cell phones for the same length of time in standby mode. Mind you, it wasn’t talking that caused participants to have trouble falling asleep because they weren’t talking on the phone – the phones were merely in talk mode. The researchers believe that the brain was agitated by the cell phone’s electrical field and had difficulty calming down so that participants could fall asleep.
So, if you are having trouble falling asleep, curb your cell phone use prior to bedtime (in addition to practicing other good sleep habits): http://www.apacenter.com/articles_sleep.asp