I suspect that all this has been planned for a long time, with the connivance of France and Germany. If you look at the last round of measures taken by the Eurozone, they are about ensuring that the banks are in a position to withstand a default, and a ring-fencing of the Euro. From the Eurozone point of view, it doesn't seem to matter what promises they make or how much money they pump into Greece, or what measures Greece takes internally, the markets don't seem able to believe in the possibility of a Greek recovery. It may be that the Eurozone and Greece have decided that amputation is the only answer.
By going down the referendum route, Papandreou is able to shift the blame from himself and his government to the unions and the people.