i went looking and found this comment at http://www.eosdoc.com/manuals/?q=500
In most operating modes, AI FOCUS is used. This means that the camera will choose ONE SHOT autofocus for still subjects, and continuous (SERVO) autofocus if the subject moves.
Unfortunately, subject motion is often not reliably detected. Use the SPORTS mode to force the camera to use continuous (SERVO) autofocus