In his consistently excellent blog, Richard Jones discusses the history of the nanobot; the "Fantastic Voyage"-style nanoscale submarine cruising our bloodstream looking for nasties like cancer cells and broken capillaries.
"Attempts to write the nanobot out of the history of nanotechnology thus seem doomed, so we had better try and rehabilitate the concept. If we accept that the shrunken submarine image is hopelessly misleading, how can we replace it by something more realistic?
Personally, I believe that we would be better served by taking a step back, and first considering the feasibility of microbots, before we even contemplate such devices on the nanoscale. Richard makes the valid point that the science fiction images of incredibly miniaturised submarines do the field of nanotechnology a great disservice, as they do nothing to dispel the myth of nanotech being "engineering, only smaller".
Based on my interpretation of Richard's book Soft Machines (he nabbed my title, the rotter ;-), I think he might agree with me that a rather more realistic (and certainly interesting) route would be to re-engineer existing living systems for the purposes of providing such applications.
As I point out in my own popular science book Genesis Machines: The Coming Revolution in Biocomputing and Synthetic Biology (Atlantic Books, November 6th -- watch this space for updates and sneak previews):
"Nature has computation, compression and contraptions down to a fine art...A human genome sequence may be stored on a single DVD, and yet pretty much every cell in our body contains a copy. Science fiction authors tell stories of "microbots" -- incredibly tiny devices that can roam around under their own power, sensing their environment, talking to one another and destroying intruders. Such devices already exist, but we know them better as bacteria."