One of the very annoying things in iCal (both in MacOSX 10.4 and 10.5) is that accept/decline notifications for meeting invitations are always sent to the sender of the invitation. This can be very annoying in some cases (for example, when a company-wide invitation for a talk is sent, the sender does not really care who attends and who doesn’t).
Fear not, there’s always some developer in the community to the rescue. In this case, it’s John Maisey’s iCal Reply Checker. After installing it, whenever you click on the accept/decline/maybe button in an invitation, a window will pop up asking whether the notification should be sent.