On January 4, 2013, Google will be shutting down several less popular Google Calendar features. One of these items is the Appointment slots. After this date, you will be unable to create new reservable times on your Calendar through Appointment slots, but existing Appointment slots will continue working for one year.
1/2/13 UPDATE: Google has changed the date for appointment slots to go away to January 30th, 2013 instead of the original January 4th timeline.
1/17/13 UPDATE: Google has reversed course on this decision and
will continue to support appointment slots in Calendar for all Google Apps for Business, Education and Government customers