The NYT has a good article pointing out how the financial collapse has led to surprisingly little change on Wall Street. Toward the end of the piece it notes efforts to require that derivative instruments be traded through clearinghouses. It later uses the term "exchange" as though it is identical to clearinghouse. This is not the case. An exchange implies that there is a market maker with an explicit bid-ask spread that can be known at any point in time. By contrast, on a clearinghouse, there is no market maker. This allows for much greater spreads and therefore much larger profits for the banks that deal in derivatives.
--Dean Baker