The most sensible single article on the Columbia loss that I've read appeared in today's New York Times. In this article, the risks of space flight are dissected. As my colleague Jeff Carlson noted, we have to hope the astronauts were given as much insight into the chance of failure and other risks as were realistic.
Elsewhere on the Net, I've seen quotes from Richard Feynman appear from his conclusions after the Challenger disaster that have similar conclusions, but this heartfelt statement from a NASA official sums it up best: Capt. Bill Readdy, associate administrator for space flight and a former astronaut, said at a news conference yesterday afternoon: "Today was a very stark reminder that this is a very risky endeavor, pushing back the frontiers in outer space. And after 113 flights, unfortunately, people have a tendency to look at it as something that is more or less routine. Well I can assure you it is not.