Iowa - Thai is simply stating that if the tires can't get a grip in the snow/ice, all of the stability systems in the world can't stop or control the vehicle. That being said, all systems are designed with the intent that there will be at least mininmal friction between the tires and the surface you are driving on (ice skating rinks, loose sand on pavement, oil slicks, all have near zero friction). To that end, I have seen a $70k land rover in Colorado hit a patch of snow covered black ice (about 3 years ago), and end up about 500 yards down the mountain... So much for VSC and TRAC on that car.
Thai - Iowa is restating what you are saying. I believe he is already assuming that there must be minimal friction, and the systems do in fact, work beautifully.
As far as systems reacting before any wheel slip occurs... Acura claims this with their MDX, but if you read the test results very carefully, wheel slip did occur and the system engaged within 1 millisecond, therefore the driver felt no slip whatsoever (and even with this system in Vail, many of the test drivers still ended up in the snow bank because of the laws of physics).
ATRAC and other related systems are phenomenol compared to what was offered before (lockers and previous gen's of TRAC), but it is not a perfect system. Don't think that will ever exist until we have flying cars, then who needs 4WD???
Thanks for the info Thai!! You are a wealth of knowledge.