Well, I attended a lecture by Dr Andrew Harrison, a profesor of chemistry from University of Edinburgh, and he talked of experiments that he had done to directly observe the effect of such radiation (rather than medical studies as is commonly done). He studied aspirin molecules and found signs of a terahertz event at gigahertz scales (translation: effect of mobile phone type waves was similar in some ways to waves of much higher frequency), and said that there had to be further study.
The answer to this question involves looking beyond standard medicine and going towards physics and biology. It turns out that the standard approach to radiation (greater frequency = greater penetration) is not the only thing involved. Both heating AND ionising effects have to be taken care of.
So: avoid talking on the mobile when u can use a landline (and its generally cheaper to use the landline anyway).