It looks like the data recovered from the downed Columbia shuttle has shown that a hole in the leading edge of the wing is the likely cause of failure and not a ceramic tile as reported earlier. This is consistent with the report referenced earlier on this site:
This report also discusses the weakness of the wing leading edges and considers significant damage to critical on reentry, noting that a hole larger than 0.63cm (about 1/4") in the insulation meets this criteria. In summary, the report estimates a one in 84 chance of failure due to orbital debris and meteorites.
Apparently, NASA engineers spent very little time analyzing the potential for failure in these carbon-carbon structures at the leading edge of the wings.