yea, right on their website, it says:
New for 2005, every deep section wheelset sports a sharp-looking, proprietary, aerodynamic dimpled pattern. But the technology is more than cosmetic. In every wind, speed, and direction Zipp’s dimpled wheels are more aerodynamic than our same wheels without dimples. In 90 percent of riding conditions, the innovation will save you between 1 and 4 watts of energy. (On an 8 percent grade, a 1-watt savings is the equivalent of a static weight reduction of 340 grams!) The aerodynamic benefit mimics the reason golf balls have dimples, aircraft wings and racecars use trip strips and vortex generators, and tennis balls have fuzz. Essentially, the dimples create an energized layer of air allowing air to follow the contours of the wheel, thus eliminating much of the low pressure wake, which is the source of 90% of wheel drag. The dimpled surface is just one more piece of evidence to our continuing passion to see how far the technology can go.