Keith Cowing is reporting that NASA sources say cracks have now been found in Columbia as well.

This may be a really big problem, though NASA isn't going to admit it immediately. If they need to replace those liners, and they don't exist, the Shuttle could be down for many months. That means there's no way to change out station crews except for Soyuz.

As I said, we've put ourselves in a position in which we have a very fragile hold on space. We are not a truly spacefaring nation, and will not be until we take some fundamentally different policy (not just technical) approaches.