Would you volunteer your back yard as a dump site for spent fuel rods? Of course not, because the invisible radiation would penetrate your dwelling, alter your DNA and other tissues and greatly increase your risk of cancer. Unfortunately, this is very likely what you are doing every time you hold a cell phone up to your head to one degree or another. 
The issue surfaced when staff and students at a school in California developed cancers at an extraordinary rate. Reminiscent of Erin Brokovich, researcher Sam Milham investigated and found that the school, due to wiring and other issues, was rife with "dirty electricity". Dirty electricity is the kind produced from computers, cell phones and TV's that are low power. This Interview with Sam Milham is very informative.
Prevention Magazine (Jan 2010) just broke this story and did a fantastic job of showing that Emag radiation is far more dangerous than previously thought and very likely causing significant cancers and other health issues. I had already been pointing out on my twitter account that one study had recently found that cell phones may cause bone loss in the hip area of cell phone carriers.  However, the damage from electrical radition is probably far more insidious than just a little bone loss here and there:
1. EMF signals (at levels even as low as wi-fi) can overstimulate the immune system and may be partially responsible for some of the allergic and auto-immune epidemic seen in modern societies. 
2. There is likely a small, but significant risk of a brain tumor just from work exposure alone. 
3. Up to 80% of childhood leukemias may be attributable to EMF exposure during pregnancy and infancy. 
This list could go on and on. However, I want to focus on something extremely dangerous: mobile phone radiation can alter the permeability of the blood-brain barrier. This had been verified in mammalian studies using standard cell phone signals and standard GMS signals.  Even worse, the researchers found that effects from this electrical radiation persisted for weeks afterwards.
Furthermore, some researchers have identified that there are "hot spot" areas where electrosmog levels are significantly higher than others, including elevators, trains, cars and airplanes.  This is because cell phones actually emit microwave radiation and in these areas the microwaves are simply reflected and bounce around undampened, reaching levels much higher than normal. Furthermore, it only takes a few cell phone users to produce these levels in many cases. So this is not something that can be eliminated by avoiding cell phone usage and is analagous to the dangers of second hand smoke.
What does this mean? It means that modern urban dwellers are potentially at much greater risk for excitotoxin damage than previous thought. Hopefully, you have read my Excitotoxin page and know that excitotoxins, such as MSG and those in Diet drinks, can damage the hypothalamus and cause Excitotoxin Syndrome. However, excitotoxins can cause their greatest damage in the brain itself. The food industry has always justified their use of excitotoxins in food on the argument that the blood-brain barrier will protect your precious neurons from damage. That, as it turns out, is a very poor assumption indeed: even cell phone usage can allow significantly greater flow of any such chemicals into the brain.
To play it safe, I recommend that you do not come with ten feet of any excitotoxin. It only takes minute amounts in the animal studies to do incredible damage to the mammalian brain and if indeed our blood-brain barrier has been even mildly compromised, the danger is substantial. There is no one except you that can guard that all-important stuff in your cranium from the food industry.
Finally, be cautious about using a cell phone directly against your head or ear. It is much safer to use the speaker phone or ear phones. Blue tooth devices are also not considered safe. Remember that there is already a study showing a tenfold risk in childhood brain tumors and exposure to higher-than-average electronic emissions.  Why take a chance?
NOTE: See my link on Cell Phone Dangers for additional information.
1) Bioinitiative Working Group, July 2007, "Evidence for Effects on Gene and Protein Expression", Z. Xu, et. al.
3) Bioinitiative Working Group, July 2007, "Evidence for Effects on the Immune System", Johannson, et. al.
4) Bioinitiative Working Group, July 2007, "Evidence for Brain Tumors and Acoustic Neuromas", Hardrell, et. al. (See also Brit J of Cancer, 2005, 93(7):842-848, "Mobile phone use and risk of acoustic neuroma: Results of the Interphone case-control study in five North European countries" and Revue d'Epidemiologie et de Sante Publique, 2007, 55(5):321-332, "Cell phones and risk of brain and acoustic nerve tumours: The French INTERPHONE case-control study"
5) Pathophysiology, 2009 Aug, 16(2-3):103-12, Epub 2009 Apr 2, "Increased blood-brain barrier permeability in mammalian brain 7 days after exposure to the radiation from a GSM-900 mobile phone"
6) Bioinitiative Working Group, July 2007, "Evidence for Childhood Cancers (Leukemia)", Kundi, et. al.
7) Journal of the Physical Society of Japan, 2006, Received March 14, 2006; Revised May 18, 2006; Accepted May 23, 2006; Published July 25, 2006, "Passive Exposure to Mobile Phones: Enhancement of Intensity by Reflection"
8) J Epidemiol, 2010, 20(1):54-61. Epub 2009 Nov 14, "Power-frequency magnetic fields and childhood brain tumors: a case-control study in Japan"