There are some excellent answers here by the agents, and, frankly, I am impressed! I am a licensed NJ home inspector for 18 yrs and have on occasion had this question asked of me. It is a legitimate concern, and, rather than go on anecdotal evidence, I once rented a Gauss meter to determine the exact electromagnetic radiation levels at various distances from a high voltage transformer. The property in question was approximately 50 feet deep and the transformer lines were running parallel along the rear of the property on the other side of the fence.
My results indicated the electromagnetic field dropped to negligible readings within 10 feet from directly beneath the power lines and were equally unreadable from the rear bedroom windows, when open. You can hire a licensed electrician to do your on testing. The following website contains useful information: http://www.healthfreedomusa.org/?p=503
I hope this help ease your concerns. I find that information in proper context, like the truth, will always set you free!