I have the Asus P5B-Deluxe and there were people using memory at 2.1-2.2V that couldn't post with it. This despite the statement in the Asus manual that it handled 13 voltage levels from 1.8V to 2.45V. In the end I went with Mushkin model 996533 which is 2.1V with 4-5-4-11 latency and DDR800. On the first attempt, it POSTed.
Other than doing something like using memory at 2.2V on say the P5B-E (which maxes out at 2.1V) I don't understand why memory compatibility is an issue for some. Does it have to do with QC ? I went with Mushkin and paid more than OCZ or Geil as they seemed to be the ones that give people the most trouble.