Desktop manufacturing is one of those ideas that sometimes seems obvious and inevitable, sometimes a bit too good to be true and sometimes completely off the planet, depending on the angle at which you look at it. If it happened, it would certainly make a huge difference, creating up the potential for open-source everything.
David Pescovitz has the optimistic version of the story.
John Quiggin 
*sometimes completely of the planet
I am sure you mean “off” the planet.
“open-source everything.”
Wait ’til people start downloading guns off the internet
Quite right, Terje. Fixed now, thanks.
downloading guns, now you’re talking. My lifelong glee at the thought of a misfortune in the howard family may be somewhat mitigated by an availability of guns on the net. Ian, what a grand idea!
I believe there are in fact already 3D CAD to rapid prototype tools that allow you to create simple solid objects out of a bit of plastic with some effort (like a 3D printer). It certainly isn’t printing an entire device, but it is still impressive.
Digging around on the web I found a link to a number of other links
http://www.caddigest.com/subjects/rapid_prototyping.htm
which might be of interest.
Well, you can already assemble (biological) viruses. As soon as someone releases the smallpox genome, that will be back. There’s no real way of stopping that, just delaying it.