coin-finder, if Jon's answer of "some cities require them" has you skittish (ie.: wondering if cities you plan to go to, has such a thing), first, let me say that this is VERY rare, for any city, in the USA, to have such a thing. I think what has happened, in this marvelous age of the internet, is that some city, somewhere, had them. And in the lightening fast info age, that gets posted. Others read it (no matter how isolated), and wonder "wow, I should inquire about permits wherever I go, because afterall 'some cities have them' ", etc...
If you're wondering is such is the case wherever you're planning to be, just be careful about finding out if such a thing is the case. Because if you walk into city halls asking if there is a permit to metal detect (which there probably isn't), it can back-fire. A desk-bound clerk can interpret your inquiry as if you are asking permission to do such activity, and simply tell you "no you can't" (as if you were asking their permission), when there is no such prohibition or rule. So the solution to this is, to look it up yourself. All cities nowadays seem to have websites. And in there, is usually municipal codes and regulations. If it is silent on the issue, so be it - you have your answer.
Ivan 
