AnandTech has a short preview of the Intel Core i7 920 with the new D0 stepping, they report these new chips definitely provide real and tangible benefits for overclockers over the previous C0 steppings. The new stepping enables higher overclocks at lower voltages:
There is no difference in application benchmarks between the two processors at stock or like overclocked settings, nothing drastic has changed like cache levels or the IMC design. Rather this is "standard continuous process improvement" update as Intel officially informed us. These improvements have enabled us to achieve higher overclocks at like or lower voltages, higher memory speeds at like or lower voltages, and as a result, our CPU, PWM, and case temperatures are down while overclocked numbers are up.