[deanotron]

The warnings comparing carbon-nanotubes to asbestos are simply because of the shape of the structure - they are both microscopic needles and our white blood cells don't do a good job of breaking them down.

Not all nanomaterials are dangerous (just as varying sizes of dust are not), but many new materials may adopt unnatural properties that our body doesn't know how to deal with. So there is still handwavyness to it all.