The UK government announced yesterday that no more homes should be built near high voltage overhead power cables, following a report suggesting that the risk of leukaemia in children is significantly greater amongst those living under or near power lines.
If it is true - uncertain still - then the mechanism remains unknown. While it could be a direct effect of electromagnetic radiation, it could also be linked to ionisation of molecules in the air around the cables.
Expect more research into this - as well as more studies into whether mobile phone radiation, or mobile phone masts also cause health risks in humans. Some studies already show quite convincing effects on living tissue and on other animals when subjected to high radiation levels for long periods. However the risk to humans is probably very small.