I realize I am adding to a thread that is almost a year old but…
I also experienced the ical unexpected crash after upgrading one of my iMacs to Leopard from Tiger. This was after having upgraded 6 other (each less than 18 months old) macs (various models) without any problem.

I found many references to this problem and they seemed inconclusive in that some things worked for one person but not another. The solution I describe below did work for me and is more specific to the Apple OS-X System data structure than the other solutions I found in that the files which iCal is upgrading are actually moved to their expected location.

I believe the problem and the solution to this iCal problem may be identical to similar problems people have reported with using other Apple applications (i.e.: iPhoto) after the upgrade.

The problem seems to be caused by a change (by Apple) in the location where Leopard looks for your iCal data as compared to where Tiger expected the data to be. Perhaps an even earlier version of OS-X was responsible since all USER DATA on the problem machine had migrated through several computers over the last couple of years. In any case, if the structure was from a pre-Tiger mac then at least Tiger was able to handle it while Leopard actually moves it’ location during the upgrade.

In earlier versions I found the data in user/library/application_support/ical but in Leopard the required files are in user/library/calendars so I moved ONLY THE DATA FILES (dot calendar files) from the user/library/application_support/ical/ to user/library/calendars.

SO THE FIX:

MOVE ALL DATA FILES (those which start with an alphanumeric string and end with “.calendars” from the ~user/library/application_support/ical/ folder to the ~user/library/calendars folder.

A final note:
It may be that the real culprit is the adding of a calendar event to your system from another system. In my case debugging revealed that an “unexpected end of file” generated the “unexpected quit” of iCal. I have seen reports by an individual who said he/she felt it was caused by the import of an EVENT which was generated by Lotus Notes on a PC/Win machine. That sounds likely to me and I have extensive programming and debugging experience as a computer professional. That’s my guess for now.

I hope this helps someone else. Thanks to everyone who took the time to share their knowledge, including those people who acknowledge it worked for them.

Soon we will have a new operating system and this will become moot. See you then with the new exciting problems! Ha!