I've heard in the past that shoes hanging from a powerline means that you can buy drugs in the area. But according to this article, in which a utility worker is interviewed, there are even more secret codes:

"The tennis shoes hanging up there mark drug areas," the worker says. "It's a sign to those who are 'in the know' that drugs are available for sale in that neighborhood." He goes on to explain the alleged meaning of yoyos and deflated helium balloons. "That meaning is a lot darker," he says. "Yoyos mean that sex is for sale in the area, and if a balloon is tangled in with the yoyos, that means both sex and drugs are for sale." He explains that the color of the yoyo indicates the ethnicity of the person offering sex.

So what are you supposed to do if you see one of these codes? Go up to the nearest person and say, 'Hey, I saw the yoyo, if you know what I mean?' Maybe objects hanging from power lines simply mean that kids have thrown things up there to be obnoxious.