This is fascinating stuff. For some things, like how locks work, the concept of "security via obscurity" simply doesn't work any longer - too many people can get the details for the secret to remain obscure. For other things - like, say, the transport of rare valuables - it's still a great idea. I love this story that Bruce Schneier quoted from one of his books:

At 3,106 carats, a little under a pound and a half, the Cullinan Diamond was the largest uncut diamond ever discovered. It was extracted from the earth at the Premier Mine, near Pretoria, South Africa, in 1905. Appreciating the literal enormity of the find, the Transvaal government bought the diamond as a gift for King Edward VII. Transporting the stone to England was a huge security problem, of course, and there was much debate on how best to do it. Detectives were sent from London to guard it on its journey. News leaked that a certain steamer was carrying it, and the presence of the detectives confirmed this. But the diamond on that steamer was a fake. Only a few people knew of the real plan; they packed the Cullinan in a small box, stuck a three-shilling stamp on it, and sent it to England anonymously by unregistered parcel post.