Recently, iCal started acting strangely with screen artifacts, non-responsive behavior, and unexpected quits (I would think that Leopard would start expecting them after a couple dozen times!). I could change views from the menu, but not from any of the buttons. Deleting the iCal preference files brought only temporary satisfaction. iCal would then open fine, but at some point, the calendars and to dos would suddenly disappear from sight. (I found this would happen when I would open the to do list.)
Hiding the to do list would make the calendars visible again, but I still could not work in them. After some testing (checking one calendar at a time and opening the to do list to see if it would freeze), I narrowed down the problem to a corrupted to do in one of the calendars. This is how I fixed it:
BEGIN:VTODO
PRIORITY:0
SEQUENCE:8
DTSTART;TZID=America/Denver:20040101T120000
UID:AF68ADF6-C8EC-4543-B0AC-0CCED4CA8FD5
STATUS:COMPLETED
DTSTAMP:20080721T221616Z
SUMMARY:New To Do
CREATED:20080827T223955Z
COMPLETED:20080812T060000Z
X-APPLE-SORT-ORDER:2127475757
DUE;VALUE=DATE:20080808
END:VTODOMac OS X Hints
http://hints.macworld.com/article.php?story=20080829092036465