Nanotech in 40k
If you like Sci-Fi and nanotech, you have to read "The Diamond Age" by Neal Stephenson.
The scariest thing about nanotech, in my opinion, would be massive swarms of nearly invisible, self-replicating dust consuming all forms of carbon. It would be an all-consuming, unstoppable cloud unleashed on a world to strip it of all life. When all the carbon was consumed, the clouds would just run out of energy (unless they were photosynthetic, which would be a bad idea) and stop working, leaving a pristine, habitable planet completely devoid of life.
Unfortunately for 40k fans, this would be such an ultimate weapon, especially if the clouds could discern DNA or some identifying charachteristics of targets, that all other forms of combat would be rendered completely useless-- except against Chaos Daemons of course! Sort of like the ultimate Destroyer Plague-- Nurgle wins!
In terms of 40k, I think that nanotechnology makes the whole use of close combat weapons make a lot more sense. Today, you cant just use a sword against a tank, but with nanovibrating (I made that word up), powered weapons you could cut through anything.