From an excellent column by Wolfgang Münchau:
The reason Greece was able to attract so much interest in last week’s bond issue was a combination of the promise of a high yield and the maturity profile of existing Greek debt. Official loans – from eurozone member states and the International Monetary Fund – make up 80 per cent of the total debt. Greece will not start to repay this until 2023. In other words the country is solvent in the short run. But long-run solvency is far from certain.
The rest of the FT piece is here. He suggests (without advocating it) that this could be the moment for Greece to default.