I use iCal every day to schedule meetings at work. Most of my meetings occur once a week at the same time, and iCal is set up to easily schedule weekly meetings. But what happens if a particular meeting is cancelled? There is no accomodation in iCal to cancel just one occurrence of a repeating event.
Here's a workaround: under the File menu, select New Calendar. Name that calendar "Cancellations" and then press Command-I to bring up the Calendar Info window. Choose red or some other color that contrasts with your normal calendar. Then, on your "Cancellations" calendar, schedule a non-repeating event that coincides with the cancelled meeting. You can even set an alarm for the cancellation.
When in the daily or weekly view of your calendar, you will see that the cancellation partially covers the regularly scheduled meeting, and the contrasting color brings it to your attention.
Mac OS X Hints
http://hints.macworld.com/article.php?story=2003013106173979