One way is to swap the interest and principle payments for the loan bank A made for the interest and principle payments of a different loan that bank B made. And banning that, or regulating it very severely rather, is precisely what is being proposed in the US banking reform legislation in Sen. Blanche Lincoln's version. The banking industry is resisting it full bore, however, since that kind of insurance trading is how so many US banks posted their most profitable years on record last year, during the worst banking crisis in 70 years.