I enjoy metal detecting and have been doing it for a few years now. But I don't find the idea of combining geocaching and metal detecting very appealing. The real fun in metal detecting is NOT knowing what you're digging up and the hope and surprise when it may be something interesting like a really old coin or lost ring, etc. Now, digging up something just to re-bury it somewhere else seems kinda silly to me. But hey, to each his own.
Also, there are many state and federal laws that specifically address where metal detecting is prohibited...and the list gets longer every year. Basically for this sport to take off all the hides would have to be buried out in the middle of nowhere or on private property with the owners permission...and I don't think many property owners will want strangers showing up on their property to dig holes again and again. And you better know the laws of your state before even thinking of pulling out a metal detector in a National or State Park, Preserve, WMA, Historical Sites, etc, etc. You can get yourself in SERIOUS trouble and ignorance of the law is no excuse.