Radiations from mobile phones is measured in S.A.R or Specific Absorption Rate. Most radiation occurs when the phone is nearest to the ear and a SAR of 1.6 Watts Per KG or less is considered safe by International Standards.
It would take another 10 years for the effects of mobile radiation be known and may be one more generation to examine the effects on future generations. Radiation is also highest near the mobile towers and people living near the towers are more exposed to the radiation. This also means, the less the signal, the less the radiation levels.
However, you can always decide to go for phone with less SAR if you experience headache or problems due to prolonged use of mobile phone.
Here is a small list of mobile phones with lowest SAR
Motorola MPx200 Â Â Â Â Â 0.12
Sony Ericsson Z600 Â Â Â Â 0.16
Blackberry 7280 Â Â Â Â Â Â Â 0.17
Blackberry 6280 Â Â Â Â Â Â Â 0.18
Nokia 9300 Â Â Â Â Â Â Â Â Â Â Â Â 0.24
Blackberry 7290 Â Â Â Â Â Â 0.24
Nokia 8890 Â Â Â Â Â Â Â Â Â Â Â Â 0.26
Nokia N90 Â Â Â Â Â Â Â Â Â Â Â Â Â 0.29
Some mobile phones with highest SAR
Motorola Bravo          1.59
Motorola Droid           1.58
Sony Satio                1.56
Sony Xperia Mini Pro     1.55
Kyocera Jax S1300 Â Â Â Â Â Â 1.55
Given the fact that 1.6 or less is considered safe, all these phone models do meet the minimum safety requirement. Even though it would be prudent to check the SAR of the mobile phones when you buy the next one soon.
Radiation from a tower is less the further away you get, correct. But beware that your phone will boost its own output to catch a weak signal from a distant tower. That is why your battery runs down quick when talking in an area of poor signal or travelling on underground trains.