Owen Callan of Danske Bank has an op-ed on this topic in the Irish Times: you can read it here.
At least so far, it appears ECB intervention has been successful in significantly lowering Italian and Spanish bond yields.
Paul Krugman has a nice little post using the idea of multiple equilibria to explain how such interventions might work, even with what could actually be relatively modest bond buying.
If your appetite for multiple equilibria models is whetted, here is a fascinating paper by Paul in which he explains the self-fulfilling crisis logic in much greater detail, though it is in the context of a currency rather than a debt crisis. The comments at the end by Kehoe and Obstfeld are also well worth reading. Interestingly, Paul is quite circumspect in the paper about the quantitative importance of multiple equilibria, putting more emphasis on steady deterioration in the fundamentals.