Sentient Developments' recent discussion of von Neumann machines and Bracewell probes puts me in mind of an essay I wrote in response to a Centauri Dreams post on the "Fermi Paradox":
Instead of relatively limited Bracewell probes, for instance, imagine an intelligent technology capable of engaging emerging civilizations in an excruciatingly patient (by human standards) form of theater. Many UFO encounters seem less like chance sightings of extraterrestrial hardware than staged events conceived by an overarching intelligence that may have little to do with the will of perceived "occupants." Just as a Bracewell probe's agenda involves instigating a simple dialogue with an emergent civilization (or at least its technological ambassadors), the more robust capabilities and resources at the disposal of a galaxy-spanning post-"Singularity" intelligence should be more than up to the task of communicating with us.
But are we confident that such communication will be limited to electromagnetic exchanges? In light of Ray Kurzweil's amply demonstrated law of accelerating returns, perhaps it's just as likely that our first conversation with extraterrestrials will take the form of a complex psychosocial experiment (in which unconventional flying objects may play only a partial role). Although undoubtedly physical, it's an open question whether "real" UFOs are metallic spacecraft in the familiar sense (although in the early days of the phenomenon researchers quickly fastened to the idea, sensing appealing parallels with our own aerospace ventures). Dispensing with the conventional notion of "mere" ET craft allows us to view the UFO problem as a manifestation of technologies ranging from von Neumann machines to "utility fog."
If the ET intent is to test our reactions to its presence (or something more profound, as the phenomenon's impact on our mythology might indicate), quickly assembling "ships" and even "aliens" from raw materials would enable the disparity of forms seen in the sky. The flexibility of nanotech construction would allow the UFO intelligence to respond to our preconceptions in "real time," thereby ensuring a permanent foothold in the collective unconscious while maintaining plausible deniability -- at least among those tasked with policing potentially subversive memes.