Daniel gave me the heads up on this nice little article - A Brief History of Computer Icons. It doesn't cover all the bases by any means but that's why it's a brief history and not an exhaustive one. Regardless, it's interesting to see the evolution on both the Mac and PC; particularly the similarities between them in terms of iconography and even color palette. There is a Part 2 but it focuses on Modern Icon Design; not quite as intriguing to me. Read Part 1.