It's become part of the language of science fiction readers and fans (its symbol can be found on shirts and buttons and keychains, for example), but it traces its roots to the original Star Trek. In that series (and later incarnations of Trek), the Vulcan philosophy of IDIC equals "Infinite Diversity in Infinite Combinations."
According to Inside Star Trek in 1968, "Infinite Diversity in Infinite Combinations represents a Vulcan belief that beauty, growth, and progress all result from the union of the unlike. Concord, as much as discord, requires the presence of at least two different notes. The brotherhood of man is an ideal based on learning to delight in our essential differences, as well as learning to recognize our similarities. The IDIC symbol is a union of a plain circle and triangle, uniting to produce the beautiful gemstone in the middle. The circle represents infinite, nature, woman, etc; the triangle can represent the finite, art, man, etc." You can see the IDIC symbol here.
Or, to put it another way, "it takes different strokes..."
And that is, I'm sure, way more than you ever possibly wanted to know about that. Live long and prosper!
PPS. The 2007 Hugo Award Winners have been announced.