contrail is a thin cloud left behind an object moving high in the sky. Typically caused by an airplane, rockets and meteors can cause them too. Anything moving fast enough or which is heavy enough will cause the thin air around it to condense, making water vapor visible. These are quite common and most people, even in areas of the country where aerospace is not the primary industry, find them unremarkable.
Which does not explain why it seems the entire blogosphere -- particularly folks from Orange County and therefore with some bleedover into the right-wing blogosphere (credit where it's due; there is an update) -- went so suddenly nuts assuming that a perfectly ordinary contrail left behind a perfectly ordinary flight, which an educated guess identifies as U.S. Air Flight 808, from Honolulu to Phoenix. There are cool heads out there in the O.C., and others who have pointed out that this wouldn't be the first time an ordinary contrail has caused a panic.
If you're interested in the subject, learn more about contrails here. If you were having fun panicking, don't let anything I've said here slow you down. But no one is in any danger from U.S. Air Flight 808. This was not a mysterious missile. This website has an image of what a west coast missile launch looks like; notice how it is clearly visible from Inyokern, a small community 160 miles east by northeast of the Vandenburg AFB launch site. My own experience -- and I'm the sort of guy who regularly looks up into the sky -- was that I haven't seen anything remarkable up there for weeks since the last F-35 test flights. There is nothing to panic about, yet people seemed eager to panic and indeed, to politicize it. I wonder what sort of free-floating anxiety is out there that fed this silly, irrational panic.