They did not skip, per se. The very first iPhone used second generation telecommunication technology. Its second iteration used third generation tech, hence they decided to add 3G to the name (3G does not mean iPhone 3). iPhone 3Gs, the s version of iPhone 3G, was the third iteration. Starting from fourth iteration, they used iteration number in the nomenclature, hence calling it the iPhone 4.
right so accord. to the nomenclature, they "skipped" 2 plain & simple.