Believe it or not, there is, in effect, such a thing in Parkersburg, West Virginia. It was shown on the PBS Nightly News. It's actually a single file cabinet (not even a fireproof one) that contains promisory notes denominated in billions of dollars and signed by the Treasurer to the SSA.
[link|http://www.pbs.org/newshour/bb/economy/july-dec01/lockbox_8-22.html|Lockbox]. Scroll down for photos.
So there! :)