DailyTech has an interesting article about carbon nanotubes, taking a look at the toxicity.
While that future may still come, there is rising concern about the potential risks of nanoparticle toxicity. Carbon nanotubes are at the forefront of the discussion. In 2004, NASA researchers at the Johnson Space Center showed that when carbon nanotubes reached the lungs, they were more toxic than carbon black and even quartz on an equal-weight basis. In 2005, researchers at UT El Paso, showed that the cell toxicity effect of carbon-nanotubes was essential identical to that of chrysotile asbestos. Last March at a Society of Toxicology meeting, researchers from Tottori University showed the first series of images that showed carbon nanotubes entering the blood within a minute of contact with the lung. Once in the blood, the negatively charged carbon nanotubes attached to red blood cells, potentially leading to future complications.