The National Association of Realtors has created a resource area called Field Guide to Power Lines.  Part of the problem with this issue is that there has been a battle of competing health studies that of course, are on the opposite side of the sprectrum.
Position: Power lines don’t affect property values
This party claims that since there is no definitive proof of a health risk, no loss in value should occur to property owners. The key driver of this movement has been the powerline industry.
American Transmission Co. 
American Trails  From an operational perspective, EMF is not much of an issue for trail activities…
Colgate Univ Term Paper  Just a term paper and not a scientific study but it concludes that there is more evidence that says there are limited health risks and on that basis, possibly not detrimental to value.
Position: Power lines affect property values
This party claims that since there is evidence that there is a health risk, a loss in value to property owners should be recognized. The key driver of this movement has been the environmental groups.
University of Missouri-Kansas School of Law  A review of a case where “…that a tax assessor’s opinion that the proposed power line would not change the assessed value of the property for tax purposes was incompetent and prejudicial…”
Wave-Group  An exerpt of the correspondence: “Late last year, New York’s highest court, the Court of Appeals, ruled that the owner of property adjacent to a utility’s high-power electrical transmission lines could seek damages for a decrease in the market value of the property caused by the fear that the power lines might cause cancer, even if such a fear was not medically or scientifically reasonable. That decision has already begun to change the outlook on electromagnetic field (EMF) litigation for utilities.”
Power Lines and Property Values: The Good, the Bad, and the Ugly  An incredibly detailed discussion on valuation approaches for powerline properties.
Realty Times Columns  Concludes that homeowners would probably pay less for a property near a powerline just because of the uncertainty.
Common Sense Application for Appraisers
In a valuation matter, where an appraiser is asked to value the effect of power lines on property values, wouldn’t it come down to how the typical homebuyer in a market felt about the uncertainty of risk? In other words, if two properties are identical, but one is located under or near a powerline and one is not and the former sells for less, isn’t that indicative of the effect on value? Whether or not EMF causes cancer or not, if a buyer pays less, it would seem to me that the difference before and after is a quantifiable measure of effect.
What do you think?