There are a fair number of people working on research and techniques to build the bottom level tools (building-blocks) for the technology of molecular engineering (the list includes: biochemists, genetic engineers, gene therapy researchers, industrial chemists, particle physicists, computer scientists, mathematicians, and roboticists).
Although only the most rudimentary devices (gas filters, small motors, atomic manipulation tools, dna computers, synthetic bio-organisms) have been created thus far, it is just a matter of time before the first "programmable devices" are built, and the first "assemblers" will follow soon after.
With the entire science being almost single-handedly founded by K.Eric Drexler (himself a student of the august Richard Feynman), one only needs to read the books of Drexler ("The Engines of Creation", "Nanosystems: Molecular Machinery, Manufacturing, and Computation") to see that he has a clear grasp of the Gray Goo concept.
The visionary spec-fic author, Neal Stephenson, provided a very lucid description of the risks of unregulated nanobots and the PRECAUTIONS taken by the "have's" (vs. the "have nots") to protect themselves from the adverse effects of hostile nano (The Diamond Age).
In short, the people working to make this a reality have already considered the risks, and will, no doubt, take steps to protect themselves (at the very least) from the adverse effects.
My advice? Be enthusiastically involved in the development of nanotech -- you'll make a fortune and ensure your own physical safety (that's my plan anyway!).
A: Because it destroys the flow of conversation.
Q: Why is top posting dumb? --clover_kicker