News of the Lehman Brothers' demise, the sale of Merrill Lynch and major concerns over the health of a range of other financial institutions has put the risk of rapid contagion and a systemic meltdown of the financial sector back at the top of central bankers' agenda.

On what was one of the most dramatic days in the history of Wall Street, investors had to digest three massive news items (alongside a host of smaller, but equally worrying developments).

First, came the news that Lehman Brothers