Cabon nano tubes are amazing. They have helped create "the new black," a substance that absorbs over 99.995% of light striking it, and they are thought to be the building blocks of structures like the space elevator or new medical devices. Just one teensie problem, they are small and fibrous. A study shows that, when inhaled, they cause inflamation of the lungs (not unlike silicates and asbestos).
Is this is the kiss of death? Actually, I think this is a sign of progress. This is science seeing the unintended consequence of another great idea and working to test for and prevent it up front. Of course, there will be other unintended consequences to nano tubes...but that is always a function of venturing into the unknown.