"The whole nine yards" is an expression I have often wondered about myself. The best explanation I could find occurred to me when I was engaged in a building project and discovered that a standard ready mix concrete truck in the US has a capacity of nine cubic yards of concrete. Thus, someone ordering a truck load of concrete delivered to his building site would be ordering "the whole nine yards," which would translate in other contexts to "the maximum amount available."

Other explanations can be found at http://en.wikipedia.org/wiki/The_whole_nine_yards.

One of the other explanations offered was that the length of the standard ammunition belt for the Browning .50 caliber aerial machine gun used in World War II was nine yards.