Looks like the ECB will act to try and calm the flight from Italian and Spanish debt. In the short term, some impact can be achieved with a relatively small outlay, but in the longer term - ie if the short-term fix doesn't calm the markets - the funds involved start to get scary, with Italy, Spain and the smaller debtor nations amounting to a third of Eurozone GDP, and Italian debt alone amounting to rather more than €1 trillion.
Krugman usefully points out that Italy has a large debt, but its deficit is far more stable than the others, and therefore ECB intervention may do the trick. In the other cases, deficits are larger and the result of economic collapse, and therefore the medium term prospects will require greater intervention.
