While iCal is an exceptionally convenient and robust calendaring program, there are some quirks to how it handles certain functions that can be rather frustrating at times. One of these is how it seems to handle invitations and automatic announcements for invitations.
When you create an invitation in iCal and include a contacts list, iCal will notify the members of that list when the event is created, and also send out reminders when the event is approaching if you add an alarm. If you delete the invitation you can do so with the option to notify invitees of the deletion; however, if you choose to edit the invitation you then cannot save the changes without sending out a notification.
These options can result in multiple e-mails being sent to invitees regarding an invitation, which may be undesirable for some people. Unfortunately, there is no way to modify this behavior in iCal, but there is a third-party plug-in for iCal called iCal Reply Checker that will allow you to manage e-mails sent through the program.
This program has two functions that I've found to be quite useful. The first is to customize which e-mails get sent, and the second is to enable editing of the e-mails in Mail. There have been times when small grammatical errors have appeared in automatically generated e-mail messages, and this option allows you to check for them and otherwise enhance the message before sending it.
Unfortunately this program does not handle e-mails that are managed in Exchange servers, but for all other calendar invitations it should be useful.