I've suggested in a number of places that it is possible - I am not committed to it being definitely true - that much of the circulation of abstracted risk which led up to the current crisis took the form of something like a horizontal Ponzi scheme. It is horizontal because it involves circulation of abstracted risk among peers, and it is like a Ponzi scheme because it involves selling something which doesn't exist. We know that Aquinas thought charging interest on a loan was usury, and therefore morally wrong, precisely because he believed that it involved selling what does not exist.
All that discursive abstractness isn't everyone's cup of tea. But if you can just imagine the Escher waterfall lined with investment bankers, lenders, homeowners, and who knows who else all dipping in their goblets for a nice deep drink, I think you'll get the basic picture.