UPDATE: I did not know how this tradition got started:
NORAD Tracks Santa, the official name of the program, began in 1955 when a Colorado Springs newspaper ad invited kids to talk to Santa on a hotline. The phone number had a typo, and dozens of kids wound up dialing the Continental Aerospace Defense Command in Colorado Springs, the predecessor to NORAD.
The officers on duty played along and began passing along reports on Santa's progress. It's now a cherished ritual at NORAD, a joint U.S.-Canada command that monitors the North American skies and seas from a control center at Peterson.
"It's really ingrained in the NORAD psyche and culture," said Canadian Forces Lt. Gen. Marcel Duval, the deputy commander of NORAD, who pitches in to field French-language calls on Christmas Eve. "It's a goodwill gesture from all of us, on our time off, to all the kids on the planet."
Very cool. Oh, and the First Lady fielded calls Christmas Eve. That's pretty cool, too.