Anyone remember the film "Them!"? The cause of the 20ft giant ants in that one was given as nuclear contamination, so you could call that a "cautionary tale" - but in fact it was just an excuse for filming screaming girls, broad-shouldered men with guns and foam rubber monsters. It was a work of fiction, designed purely as a spectacle, with no basis in fact.
Replace "giant ants" with "grey goo", and "nuclear contamination" with "nanotech contamination", and hey presto! you have a Michael Crichton book! A B-movie work of fiction, leave-your-brain-at-the-door entertainment.
If you want a better example of nanotech in action, check out Neil Stephenson's "The Diamond Age". It's a bit slow, but it has plenty of new ideas, and it's a whole lot better written than Crichton's stuff.
FWIW, Donuel is right that caution is needed with nanoscale particles. The high surface area and high mobility of particles means that how a substance behaves in a large mass might be rather different to its behaviour when it's in nanoscale particles. The size factor also becomes an issue when they can penetrate cell walls.
All this is just about nanoscale particles, though. A nanoscale particle is to a nanomachine what a rock is to a silicon wafer - it might be the raw ingredient, but *boy* is there a lot of work needs doing on it! :-) As far as nanomachines go, we haven't invented the wheel yet.