The idea of a non-biological self-replicating system is not new. John von Neumann proposed his concept for such a system back in 1948, detailing a machine that could use raw materials to build another version of itself and duplicate its programming into the new machine.
Granted, this was more of a thought experiment at the time, but now we are rapidly approaching the point at which our technology could make it a reality.
The benefits of self-replicating technology are certainly tempting. You would only have to manufacture a small amount yourself before the machine took over and costs of things like shipping would be slashed. The problem we have is one of handing over certain powers of judgement and creation to something with much simpler programming than us, particularly in the case of nanotechnology.
This is easily the kind of technology that you can lose control of and, even if the machines created are not harmful or dangerous themselves, the rapid consumption of resources in exponential replication could well lead you your robotics lab being disintegrated around you by a swarm of hungry nanobots. The "grey goo" scenario might be extreme, but not totally off the wall.
If the machines themselves are harmful, however, we run into a whole new set of problems...