Strategia and Renegade make the point that "life" in our galaxy need not be of a sort familiar to us, i.e. carbon-based. This is certainly valid (within the limits of our knowledge), and makes irrelevant any probability calculations requiring Earth-like conditions. Of course it's only natural to look for more-or-less familiar life forms first: we already have one example of a carbon-based ecosystem, but exactly zero examples of any other type.
What forms could "unconventional" life take? I vaguely recall a juvenile science fiction novel which featured Mercurian creatures that resembled ball lightning. Arthur C. Clarke wrote a short story ("Out of the Sun" ???) in which an electromagnetic entity of some sort was ejected from the sun in a solar flare. Silicon seems to be a popular basis for sci fi life forms, probably because of its position just below carbon in the Periodic Table. If I recall correctly the "Horta" of Star Trek fame (episode "Devil in the Dark") was silicon-based.
Nevertheless, silicon is not carbon. As I recall from my prehistoric chemistry classes, silicon's 4 valence electrons are one electron shell "out" from carbon's, giving the elements roughly similar but far from identical chemical properties. For example, silicon does form polymer chains (as the bosoms of countless Hollywood starlets attest), but I've read (I'm no chemist) that these chains don't approach the length and complexity of carbon polymers (DNA, anyone?). Recall also that silicon dioxide is a solid at temperatures that vaporize carbon dioxide. Now some earth organisms (e.g. diatoms, sponges) take advantage of that and use silicon dioxide STRUCTURALLY, but on Earth at least evolution has favored carbon chemistry for life's vital functions, despite the greater abundance of silicon.
Incidentally, as a naive layman I wonder if a detailed study of the ways these creatures build their silica frameworks might suggest how a hypothetical silicon-based life form could "work".
A couple of places on the web with some tidbits about silicon:
http://en.wikipedia.org/wiki/Silicon_dioxide
http://www.webelements.com/index.html (click on the elements)
Re: OT: Extrasolar planets discovered directly
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