Anyway, Luskin promptly demonstrates a few things. First, he does not quite grasp the meaning of "almost," declaring that Vogt had "no doubt" that there was life on Gliese 581g. Granted, Vogt contributed to Luskin's confusion by giving the chances of life at 100%; still, the distinction between "no doubt" and "almost no doubt" approximates the distinction between almost falling off a cliff and actually falling off it.
Second, he does not quite grasp Vogt's argument. It is not, as Luskin puts it, "Dr. Vogt has "no doubt" that life evolves and exists elsewhere because he knows that it evolves and exists elsewhere." Rather, Vogt assumes that because life is common and adapted to some rather extreme environments on this planet, it would arise on and thrive on any planet which had similar environments to those that harbored life on Earth (and while Gliese 581g is thought to be "Earthlike" in only a limited sense: three times Earth's and keeping one face to its sun at all times, it might indeed have niches not much more hostile than some that bear life on Earth) (assuming, again, that it meets one particular criterion that Earth does, actually existing). Vogt doesn't actually explain why he thinks life is likely to arise on a suitable planet in the first place; I would guess that he reasons (as others have) that since life arose so early on Earth, abiogenesis is not, in fact, .very improbable (or else perhaps he thinks life arose in space and spread throughout the galaxy from wherever it started).
Third, Luskin seems to have some slight problem differentiating between the position of one "Darwinist" and another. Accepting evolution doesn't involve accepting any particular idea about the probability of abiogenesis (except perhaps that it has to be at least as high as "once per universe," or at least, that however unlikely it happened at least once). "Darwinism" does not depend on any particular idea of how or where life originally arose (strictly speaking, it does not depend even on the assumption that abiogenesis was a natural phenomenon). As P.Z. Myers noted on his blog Pharyngula, we don't have any idea what value to assign to the chances of life arising on another planet -- and one might suppose that Luskin has both heard of Myers and regards him as a user of "Darwinian logic." Vogt's reasoning is his own; it is not "Darwinian" to a greater extent than Myers'.
Of course, Luskin's incomprehension may well be tactical. He wants to demonstrate that "Darwinists" make hasty, unexamined assumptions (about common ancestry, about the efficacy of nonmagical causes, about the likelihood of life in space), and picks an extreme example (which he then rather misrepresents) to make his case.
There is another point, though. Luskin goes on to state that the theory of intelligent design (that would be "somewhere, sometime, somehow, Somebody did something, and furthermore, evolutionists are wrong about something or other") is compatible with life on other worlds. He just wants to make sure that the problem is approached scientifically (which I would suppose means that if we find extraterrestrial life, we declare that this is evidence for design, and if we don't, we declare that that is evidence for design).
I'm not sure whether he's hedging his bets against the chances that scientists actually will find extraterrestrial life, or whether he thinks that such a thing is to be expected. After all, if you think that the universe was designed for a purpose, and that life was designed for a purpose, one might suppose that the Designer wanted to make a universe that had life in more than one out-of-the-way planet (and of course "life" is not the same as "intelligent life," and does not raise the same theological conundrums such as "why are extraterrestrials being punished for Adam's sin?").
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