City status relates to the importance of a place - in times gone by it used to having a cathedral that gave this importance, then being big and industrialised, now it's strategic importance that should be the clincher for Dover.
In a more light-hearted vein - Brighton was created a city by the combining of Brighton and Hove, two separate towns previously in adminstrative terms. So maybe Dover with the port and Folkestone with the channel tunnel could combine to form a new city in the same way. Just imagine the arguments over the name
