I'm too darn lazy to actually read all the posts in this thread, so if I repeat what others have said, well, sorry.

Here's my biggest fear about nanotechnology: evolution. As Issachar said, nanites that aren't self-replicating are essentially useless. However, any entity that engages in self-replication evolves. Mutations aren't limited to organic beings--any machine that did nothing but copy itself over and over again would eventually develop mutations. Maybe a sunspot would knock out a key portion of it's central "brain" while it was trying to duplicate itself, and the "duplicate" it made would end up up being slightly different from itself. That's a mutation. And random mutations are what drives evolution. Even if you don't believe that evolution is what created the vast array of species on Earth today, you can't deny that the *existence* of evolution by random mutation and natural selection is a sound principle. There are any number of recorded examples of such micro-evolution, and if I wasn't such a lazy bum I'd look one or two of them up.

And given that the lifetime of a nanite would most probably be very short, replication would cause each nanite "generation" to be a very short time period indeed, perhaps on the order of days or weeks. With that rapid development, evolution would also be incrediably speeded up. I don't want to release a nanite into my body that is supposed to eliminate diseased or dying cells that might through the course of a few "generations" lose the ability to distinguish between diseased cells and healthy cells, and just start killing them all.

-- Dave