Does what's outlined [link|http://www.richardsnotes.org/archives/2005/05/01/tiger-upgrade-issues-with-ical/|here] help?
Someone had similar problems to you and was only able to fix it by doing a [link|http://www.codecomments.com/archive240-2005-11-683450.html|fresh install without preserving the user's settings].
There's a dearth of information out there on solutions to problems like this, as I'm sure you know. I hope you get it figured out.
Cheers,
Scott.