FYI, EMF stands for ElectroMotive Force.. or a fancy name for voltage. What you may be talking about is EMI (Electro Magnetic Inteference) or EMR (Electro Magnetic Radiation) and they don't actually cancel out, they are always there. Usually due to leakage current and that sort of thing, that is always present in high-voltage/high-current carriers like power lines. It's the leakage, usually some sort of arcing that causes problems (like thunderstorms with lightning).

There also was something discussed within the AMA called BPL or Broadband over Powerlines a few years ago that was investigated pretty thoroughly. The result was that it would not cause problems. I still think the jury's out with this one, especially for 72 MHz.