If the advent of a technological Singularity on Earth is inevitable and if more than one technological civilization exists in the universe, it follows that there must be more than one Singularity. If some of these plural Singularities involve the uploading of sentience, then sentience may be broadcast as a "file" on light. Earth may be bathed in this modulated light which passes by with no effect until such time as the Cybereon we are building develops sufficiently to act as a substrate for the sentience encoded in the light. A file of sentience, able to recognize the sufficiency of an emergent cybersubstrate, inserts itself into it as if an interstellar computer virus.
(Via Variable Gravitas Content.)
You know what would be really cool? If these luminal viruses carried plans to build devices by which they could migrate to the nervous systems of a host species -- a sort of "Invasion of the Body Snatchers" upgraded for the cyber age. And wouldn't it be hilarious if the first recipient of an interstellar information virus was one of SETI's "can't get here from there" crowd? (Oh, to have Seth Shostak carted off to Area 51 where we'd be spared his insufferable wit.)
Rudy Rucker addresses a similar scenario for galactic travel in his novel "Saucer Wisdom," in which Fermi's "paradox" is resolved when an eccentric UFO contactee learns to decrypt omnipresent encoded aliens.