I think having a mine or any other important area as a PvP zone is a bad idea...not everyone wants to have to worry about getting whacked by other PC's while prusuing their crafts or trying to fulfill a quest...especially noob players who would have no chance.
If anything.....perhaps certain areas along the roads between cities could be PvP areas to allow for highway bandits...but players could go around it if they wanted to waste the time and energy...or other players could offer services gaurding travellers in that area.
Perhaps the Thieves Guild could allow for temporary "Permits". One could buy a permit that would last a certain amount of time...and would receive some token. They could then carry this token to an area and "plant it". A fixed zone around this token would become a PvP zone for a fixed length of time, or until the death of the character who has the permit (thus allowing for bounties). The token, once planted, could not be moved.
Of course, if a player plants his token too close to the gold mine, he would be swiftly dealt with by the other players, I suspect, so the thief would have use some guile in where he decides his zone of banditry should be.
