Before Tiger (and iCal 2.0), it was possible to manually set the status of attendees in iCal. This meant you could set an attendee (or an event) to Confirmed, Cancelled, or Undecided, and an icon in the upper right of the event would indicate the current status. iCal 2.0 introduced automatic attendee status: now when you send out event invitations, the recipients reply with specially crafted emails to indicate their status. The problem is that you can't manually modify that status any more.
It turns out, though, that the status is AppleScriptable. So I wrote a small script that allows you to set the status for individual attendees to a given event. Copy and paste the text into a new script window. Then just save it into your user's ~/Library/Scripts folder, turn on the Script Menu, and voila, set status to your heart's content.
[robg adds: I haven't tested this one.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060831192659694