For a full-scale test, Adam and Jamie put a layer of clay on a car and did two more sets of runs on their track – one with a smooth clay surface, the other with dimples pressed into it. The respective fuel efficiencies were calculated as 26 and 29 miles per gallon. Although the original myth was invalid, the theory behind it was sound, leading to a final judgment of “Busted, Concept Plausible”.
Discussion threads on several auto forums discussing the Mythbusters episode note that "shark skin" textures on military fighter aircraft (and on America's Cup yachts) serve the same purpose, that dimpling on the undercarriage of some Lexus cars reduces noise (by reducing friction), and that textured paint is banned on professional race cars.
Mythbusters achieved the dimpled effect using modeling clay applied to the surface of a Ford Taurus. It's not clear whether the same effect could be achieved with a ball-peen hammer.
Screencap credit. A brief YouTube video of the car (not the full episode) is here.