Not just the foam strike
There were a great many lessons in the Columbia disaster. Whilst el Reg provides a nice write up the basic reason, taking time to look at why it could happen, as well as the what happened would be worthwhile.
The investigation uncovered a huge number of flaws in management of the shuttle programme. It wasn't just that NASA lost a second shuttle that set in motion the retirement of the fleet, but that NASA manifestly was not able to show that it was up to the task of managing the programme. It was clear that NASA would never be able to get the shuttle programme past losing one in every 50 flights. Some of this stemmed from inherent defects in the shuttle's design, many of which were inflicted on NASA due to the politics and budget cuts in the 70's, but a great deal from issues in NASA's internal culture.
Mission rules required that the ground control team provided constant oversight of the mission. Yet there was so little concern about the state of pay that the mission controller gave the team the weekend off. Both violating mission rules, and evidencing the total lack of interest in the foam strike.
Whilst the foam strike was always the prime suspect in the loss of the orbiter, there were other very serious engineering flaws uncovered. The investigation spent some time specifically looking at NASA's processes, and specifically criticised it's "broken safety culture." The external tank manufacture had been so tightened up financially that the position of manager of a particular part of manufacture, and the position of safety and quality control for the same part was occupied buy the same person. Yet no-one seemed to realise the fundamental conflict and inevitable loss of safety this would bring. Ultimately NASA was shown to have not learnt any lessons from the loss of the Challenger. The same hubris, and culture of "we got away with it last time" that doomed that craft, also doomed Columbia. The issue of foam strike was degraded from a flight critical one - where in the original rules for the orbiter this was a non-negotiable flaw that would have led to instant grounding of the fleet until resolved. It was let slide to the point that it was considered a regular "problem" that they would ultimately sort out, and not considered a serious enough to impact flight. An identical mindset as they had for the SRB O-ring seals that doomed Challenger.
The report on the disaster is worth reading from cover to cover. Whilst there is nice story of forensic engineering, the real story is in the surrounding culture, and the question of just how and why it was allowed to happen.