That actually came from the original C2 Wiki, although it's been refactored (across sites apparently...it's now on MeatBall) significantly since I first came across it c. 2003:
I suspect it's a much older idea than that, probably dating back to the Enlightenment. It's closely related to the Principle of Charity, which was coined in 1958 but again, refers to a much older principle of logical discourse:
I think the Principle of Charity is pretty spot on for 1958. I suspect John Rawls said something assuming good faith, too, given the quotes I've found [1]:
In his book A Theory of Justice, Rawls stated that citizens are obliged to act in good faith, and to assume good faith on the part of others until clear proof emerges to the contrary. They must recognize, in effect, that the system cannot meet everybody's claims at once and accept that at times they will be on the losing side.
And a good recent blog post [2]:
Several maxims guided me in doing this. I always assumed, for example, that the writers we were studying were always much smarter than I was. If they were not, why was I wasting my time and the students’ time by studying them? If I saw a mistake in their arguments, I supposed they saw it too and must have dealt with it, but where? So I looked for their way out, not mine.
http://c2.com/cgi/wiki?AssumeGoodFaith
I suspect it's a much older idea than that, probably dating back to the Enlightenment. It's closely related to the Principle of Charity, which was coined in 1958 but again, refers to a much older principle of logical discourse:
http://en.wikipedia.org/wiki/Principle_of_charity