I think you're talking about the "grey goo" problem. There's a couple sceenplays in the works about that already.
Please quit trying to discourage me. This isn't the typical "grey goo" scenario, this is the "black filament" scenario. These are living (perhaps intelligent?) nanotubes which just seek out carbon atoms; they could care less about any other kinds of atoms. It's merely our tough luck that we're a carbon-based lifeform.