Wow, did this thread ever go off course.
I didn't see where the OP asked anyone about the various stereotypes of various materials. It is a subject that can be, and should be, argued on a different thread. This isn't the place. Why not put your respective ego's aside, and get back on topic, which in this case, is whether the LBS salesman was telling the truth about doubling the distance.
I personally have never ridden a carbon frame bike, but I can state categorically that the claim of doubling the distance, based on frame material alone is completely absurd. In this case, I find it to be offensive, and the bike shop was trying to make a sale (and line their pockets) based on exactly the same stereotypes that are being argued. I would let the manager know that he had lost a sale, and take my business somewhere else. There are other ways to make a sale without relying on blatant dishonesty.
There is nothing that will "double the distance" ridden with the exception of the will of the person doing the riding. Is one frame better type better than another? Maybe, maybe not. In any case, that wasn't the question asked.