I'd just like to clarify something. PaulK, when you said:
"However so far as I am aware science has NOT concluded that any unknown intelligence is involved."
do you mean ANY unknown intelligence (human or extraterrestrial), as I had meant in my OP, or do you mean an intelligent species that we have not yet met (i.e. extraterrestrial)?
I ask, because if we are both talking about any intelligence, then your post reveals the same dilemma that I tried to underscore in my OP. You stated that:
"it is true that most [crop circles] are considered the result of intelligent work."
So as your statements imply, it seems to be reasonable and in fact intuitively obvious (by looking at crop circles) to conclude intelligent design, but science has not definitively concluded intelligent design. We can take this a bit further, and say that
a perfectly reasonable conclusion such as intelligent design cannot be verified scientifically.
This leads me to think that a reasonable conclusion may not necessarily be possible to investigate by scientific means, and yet this impossibility doesn't detract from how reasonable the conclusion is. It also leads me to wonder how reasonable the intelligent design theory really is, if you momentarily ignore the fact that it is obviously non-science.
It also makes me wonder whether, given future advances in technology, the so-called theory of intelligent design may someday become a valid scientific theory. If SETI scientists are able to someday invent powerful telescopes and other devices that can detect and observe aliens actually drawing crop circles in our fields, then the study of crop circles may fall within the realm of true science (exobiology?). Likewise, if we can invent what may now seem to be impossibly powerful detection techniques that allow us to peer into the past, we may be able to verify whether or not an intelligence has guided evolution.
But of course, if we did develop such techniques, then we would probably end up falsifying intelligent design at last.