The answer, courtesy of SteveRadick.com
"It all started back in 1955 after a Colorado Springs-based Sears Roebuck & Co. advertisement for children to call Santa misprinted the telephone number. Instead of reaching Santa, however, kids ended up calling NORAD’s Commander-in-Chief’s operations “hotline.” The Director of Operations at the time, Colonel Harry Shoup, realizing that kids were relying on him for quite possibly the most important information of the year, regularly checked the radar for Santa as he made his way around the world delivering presents. All of the children who called were given updates on his location…and a 50 year old tradition was born."
It's certainly nice when mistakes lead to a happy outcome.