>I don't think a civilisation capable of interstellar travel is going
>to engage in building megalithic structures on planetary surfaces. As
>far as we know interstellar travel requires micro craft built with
>nanotechnological means. Shortly after arrival it autoamplifies into a
>vast machine ecology living in space as their native habitat. There is
>no point for them to descend down the gravitational gradient for
>purposes other than large-scale mining (leading eventually to
I don't think this is the _only_ sort of alien technology we should be looking for, but it's certainly a good scenario. List members tend to assume that "exotic" ventures like interstellar colonization will necessarily rely on all kinds of fancy nanotech, but don't forget that the idea of traveling to other stars isn't as exotic as it used to be, in a way. If our history had gone just slightly differently (and I'm not refeering to our space program, but to world history in general) we might have already sent out craft using ramjet and ion proulsion.
While the nano-alien scenario is intriguing, I'm not ready to say we've never been visited just because we don't see any evidence of this specific kind of visitation.