Scientific skepticism goes all the way back to attitudes about reason and knowledge that came out of the Enlightenment. Scientific skepticism in its' current form traces back a few decades with perhaps Paul Kurtz, Isaac Asimov or Martin Gardner plausibly staking claim to the modern skeptic movement. More formally, I would pick 1976 when CSICOP coalesced.
http://en.wikipedia.org/wiki/CSICOP#The_formation_of_CSIOn edit: I left out Marcello Truzzi and Ray Hyman in the list of people who could be considered as progenitors of the modern skeptic movement.
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