Once nanotubes start to reproduce themselves, they will take over the world. They are 600 times stronger than steel -- nothing can stop them. They will consume human flesh and "mine" all living material, be it plant or animal, for the carbon atoms. They'll insert their tubular tendrils into our bodies while we sleep. No place will be safe.
But enough! I better get to work on the screenplay before someone steals my idea.
I think you're talking about the "grey goo" problem. There's a couple sceenplays in the works about that already.