Edit: The most "Fair" acchievable is to probably have Win 7 as the only Os to skin.
Some skinners dont have win 7, have never skinned it... some users dont have XP, some dont have 7..
Seems to me, the only fair way to do it is to leave it open.. have the skinner skin what they want\can.. and let the popular vote decide who has done the best job on one , or all three.
Lets face it, skinning all three is MUCH more work.. but also leads to more opportunity to please more users.
Whatever the rule ends up being, I am fine with.. I just hate to neglect a (still) large portion of users. (XP)