Eric Drexler has some useful words at the Foresight web site.

The whole thing is worth reading, but I particularly enjoyed what is yet another interesting take on what happened on Flight 93:

#3 Our advocacy of openness as the safest strategy has been validated. In under two hours, the problem of airliners hitting buildings was solved ? by passengers in the fourth plane to be highjacked. They did it "open source style": shared information on the need, collaborative design, and unpaid group implementation. (With earlier information, they might have been able to save their own lives, as well as those in the building their plane was meant to hit.) Their example can inspire us as we work to find a "bottom-up," distributed, networked, immune-system-style defense against the abuse of nanotechnology.