I'm not surprised that insulation tiles appear to be the smoking gun in the Columbia disaster. What's striking is that after the loss of the Challenger in 1986 that engineers could realize the dangers of another failure and watch history slowly repeat itself over the next 18 years. Significant damage from foam delamination had been identified as early as six years ago and yet nothing was done to prevent this kind of failure. I believe that we'll see the same kind of deer-in-the-headlights effect that gripped NASA even after they knew that the o-rings in the booster rocket were a severe design flaw. There's just no political value in grounding the Shuttle fleet for years while you redesign major system components, is there? No, you've got to keep flying, even if it means foolishly risking the lives of brave men and women.
When this investigation ends, we will see a similar conclusion to 1986: that it was a crapshoot, and the every time they launched they were just hoping that flying insulation didn't cause too much damage. It will be clear that NASA knew the possibility for failure but did not want to disrupt the program by addressing the problem.
In a related note, the delamination apparently became a more significant problem when Freon, an ozone-unfriendly substance, was removed from the insulation manufacturing process. This apparently made the insulation more brittle and prone to fracture. It's unsettling to think that attempts to make the Shuttle more "green" indirectly led to 200 tons of radioactive debris raining down on the southwestern United States.
Update: NASA is downplaying the insulation as the root cause. I can understand this, as they probably don't have a model that explains how light foam could cause all that damage, but I think this also might be a little P.R. to keep people from lynching NASA until the investigation actually gets underway.