I'm an avid cinema goer and I recently watched GI Joe: Retaliation and in that film they had a weapon in orbit which basically dropped large tungsten rods towards the Earth, these rods only had gravity behind them and weren't rocket propelled in any way. One of these rods dropped and hit London and wiped it out.
How realistic would such a weapon be?
What speed would the rod reach in Earth gravity? and would a rod be capable of such devastation?