An interesting new paper by Milan M. Ćirković and Robert J. Bradbury suggests the problem with SETI is that it's searching for the wrong thing. The paper is titled Galactic Gradients, Postbiological Evolution and the Apparent Failure of SETI (PDF format). The authors note that everything we've learned in the last few years makes Fermi's Paradox more distrubing than ever. Common explanations for the lack of ET is that Earth is unique in some way or that all intelligent civilizations self-destruct relatively quickly. This paper propose something else. We've been looking for biological intelligence. The authors believe it may be normal for any intelligent race to evolve into non-biological, information-based life forms. The authors draw on a wide range of sources to theorize about what an advanced technological race that had evolved into machines might look like. For example, machines would likely want to locate in the coldest regions of space to achieve maximum thermal efficiency. The authors note that these ideas were initially explored by SF writers.