MASS is the key. For most of history, warfare consisted of two groups of people hacking at each other, and the *more numerous* one (with some allowance for weapons and tactics) stood over the heap of corpses at the end and claimed victory. The scary thing about technology, nano in particular, is what I call "the disproportion of means and ends": one man's finger on the nuclear launch button; a few vials of virus and some plane tickets; releasing a single "gray goo" replicator -- and it's all over for everybody.
We need to take the threat of nano warfare seriously. Providing we do,
there's no cause for despair, because,
IT'S THE MASS, STUPID!
I want to live surrounded by nanites; so everything around me behaves
just as I want it to. Even the dirt in my front yard will aerate
itself, fall cleanly off my feet, etc.
Whatever its job, each of my nanites has a "Swiss Army" program: "disassemble any unfamiliar nanites and sound an alarm." Because my "benign swarm" permeates every object I own including me, my furniture, trees, etc.; there won't be "poorly-defended energy sources." So if you want to goo me, you'll need to transport a really large MASS of replicators to my vicinity, and finish the job before the people and robots around me swamp them. You'd do better with a gun!
Even if you do me in, the alarm will circle the planet before you disassemble my epidermis. A local replicator attack will be just that: local. "Madman Kills Neighbor with Goo." Every object on the planet will swarm with nanites, and each object and its nanites will have a registered owner, with dust-mote cops checking their IDs and configurations by the second. Remember large-scale goods distribution will be almost extinct; there won't be excuses for distributing mass quantities of anything. (Insidious possibility: it's a design downloaded to your own household assembler; we'll need virus checkers, etc.) Remember,
IT'S THE MASS, STUPID!