Bob,

Here's a copy of Buffet's letter on that topic. It helps but the entire derivatives scene is a story of taking risks on someone else's risks which were taken on someone else's risks. He also criticizes the inherent tendency to be optimistic, coupled with derivatives reporting earnings today whose full costs cannot be known until some tomorrow on the horizon.

WB returns to the topic in 2005 giving figures for the amount lost. Bear in mind that these losses are accruing to a well managed company winding up positions out of choice and in its own time. What could happen if such things are forced out was foreshadowed in the LTCM crisis a couple of years ago.
Whoops, I should add that its on Page 9 of the pdf, Page 10 of the report