Well I can tell you that the effects of magnetic fields on developing embryos are well documented for one biological effect. TMS shows that magnetic fields can affect cognition as well. So we know magnetic fields affect biological systems, the question of course is one of field strength (and direction) allied with long term exposure.
My wife administered an epidemiology study which measured levels in homes and sought a correlation with cancers. I can't remember what the conclusions were, but I'll ask. It was of the 'did the electricity meter on the other side of the wall from the bed cause my kid's brain tumour?' type.
Personally, I wouldn't live underneath high tension lines, but then I earn enough so I don't have to.
Peter