I think the reply to this is rather simple. We do not know of any natural phenomena other than humans designing the crop circles that explains why they are here... After all, we know humans like using symbols and whatnot to communicate. It's not all that unlikely that symbols appearing in a crop field is the result of human action.
On the other hand, we do have a natural explanation for the design inherent in life. It's called evoloution. Evolution does not involve intelligence, but it sure as heck does design. Evolution even leaves certain signs to indicate that it has happened. We can see these signs in life.
Inferring design in crop circles is simply submitting to the fact that we don't know of any other plausible ways it could have happened (I don't see how the crop circles could have been created by natural selection and random mutation for instance). Of course, if someone submits an alternative more likely than human design... bring it on. I'd like to see the reasoning, however, that makes extra terrestrial design more likely than human design.
This message has been edited by Maxwell's Demon, 01-29-2005 21:45 AM