Takuan Seiyo had an important article in September called The Case Of The "Disappeared" Subprime Minority Borrower,in which he explained how Wall Street came to be invested in bad mortgages on a large scale, and what "Mortgage-backed Securities" and "Collateralized Debt Obligations" are, as well as what Wall Street means by a "tranche." This is a graphic representation of the same idea: 
