The #ie6countdown is a step in the right direction, but you need to do one more thing. Those users who will not or cannot upgrade from XP can only get up to #IE8, so even if every single Windows XP user upgraded, they will not be able to go beyond #IE8 leaving them years behind everyone else. #IE8 will become the new #IE6.
Instead I ask that if you cannot offer these poor souls a continuing upgrade path that you offer them a browser which can. Safari, Chrome and Opera all still support XP. If you cannot continue to offer them updated versions of IE then direct them somewhere else (or at least offer them Chrome Frame)