Sorry for posting a graphic you'll have to click on to blow up. The blue circles are the market valuations of the banks as of Q2 2007 - and the green circles (yes, there are some - you'll have to look closer) are the market valuations 8 days ago on January 20th.
I've taken the graphic from Free Market Fairy Tales.
Addendum: I've just read today's contribution from Peston,who is predictably in Davos:
"As I write, the chief executives of many of the world's big banks are meeting in top-secret, private session - to discuss what to say in their also top-secret meeting with finance ministers on Saturday morning. Here's the scoop on what the bankers will say: "Help!!!!!" (roughly translated as "only the further generosity of taxpayers can prevent us falling over as a consequence of the big losses we're incurring on our imprudent lending"