In the FT today, David Roche puts meat on the bones:

And finally, there could be counterparty losses if financial intermediaries in the market go under...The most likely place for this to happen is among financial guarantors, the so-called “monoline” insurers...If the monoline guarantees on bonds and credit derivatives were to be removed, the rule of thumb is that every 1 per cent decline in the price of insured bonds would give rise to $10bn of losses on bond portfolios elsewhere in the system. We estimate bond portfolio losses of $150bn-200bn were this to happen – or equivalent to the impact of the subprime crisis on the US banks.
Much of this pain (loss) would have to be absorbed through the CDS markets as additional losses to the cost of defaults. Also, the decline in credit quality would also hit CDS prices to the tune of about $40bn-$50bn. In total, we estimate that global losses in CDS markets and the underlying credits they insure would be $365bn-$425bn. That is about equal to the losses likely to accrue from the current crisis in subprime and other pools of debt for global banks and non-bank financial intermediaries...

The monolines have been looking unsure for some time now - we wrote in December that they were undergoing a crisis. How soon till the first one collapses? Not long.