The Economist's Gulliver blog sums up the unfortunate problems with Boeing's biggest project:
The latest delay looks like the most serious yet. In May, routine bending tests in the workshop showed the wing structure to have separated from its skin ("delaminated") at 120%-130% of the load limit. To pass muster with the Federal Aviation Administration and other certification bodies, wings have to sustain at least 150% of the load limit without rupturing.
The problem...has been identified in the past and recognised as a problem. The issue has arisen on other composite airplanes. Indeed, the stress point at the end of the 787 stringers showed up as a 'hot spot' in Boeing’s computer models before the delamination in the wing bend test—but for some reason was never addressed.
It's worth a read, as are the articles Gulliver linked to.