For more than 50 years, the North American Aerospace Defense Command (NORAD) has been tracking jolly old St. Nick as he flies around the globe on Christmas Eve. The story of how it all got started is pretty amusing.
Apparently the good folks at Sears got the phone number wrong for a hotline to Santa in one of their advertisements. Instead of getting through to the elf himself, kids wound up talking to the NORAD commander-in-chief. After a few kids called him with their wish lists, the officer figured out what happened and instructed his staff to begin tracking Santa. From that point on, children calling the hotline would be given updates on his location, and a new Christmas tradition was born.
Of course, it's not all about the kiddies. I figure that Santa probably has a good relationship with the controllers on the ground to help route him around other airborne traffic. His entire flight is conducted in near total darkness, after all.
Swing by the NORAD Santa tracking web site for more on the story and some nifty extras like a Google Earth download that will show Santa's flight in 3D.