Just how much are those dodgy assets worth?

Not a lot it seems. According to Gillian Tett in last week's Financial Times we are starting to see what the collateralised debt obligations (CDOs) which are largely based on US mortgages are really worth (the things that are at the root of the crisis). A large number of those issued from 2005 are already in default (the issues have not kept up repayments). Of the ones that have been liquidated, it seems that the "super-senior" ones, the safest of all, are only paying back 32%. Of the slightly riskier "mezzanine" ones - which still received a AAA rating apparently - the recovery rate is only 5% (what is got back from the initial face value of the asset)! So much for the ratings agencies.
Gillian Tett, an FT journalist, is famous for having predicted the disaster. Here is an interesting interview with her. The bad news is that she studied social anthropology, not economics, and spent a year milking goats in Tajikistan, and apparently this is what helped her figure out what was happening.