An extraordinary day that signals a seismic shift for the US banking system.

First, we have Lehman Brothers, Wall Street's fourth biggest bank, making its announcement here that it intends filing for bankruptcy. This is a a 158 year old bank that survived railroad bankruptcies of the 1800s, the Great Depression in the 1930s and the collapse of Long-Term Capital Management a decade ago. The news came after bailout talks collapsed.

Then Bank Of America agrees to buy Merrill Lynch for $50 billion.

So suddenly, two of Wall Street's biggest banks are headed extinction. Add to that, AIG now seeking a $40 billion bailout.

The events have inspired BusinessWeek to describe it as Wall Street's "perfect storm" but this is not the end, there's more to come. Former Fed chief Alan Greenspan has warned that "we will see other major financial firms fail". Meanwhile economist Nouriel Roubini has warned we could see a run on the investment banks. "It's clear we're one step away from a financial meltdown," Roubini said.