Just a quick comment...
You've "re-installed Leopard several times" --- why would this be necessary for one application failure? Please explain why you beat a fly with a hammer?
Are all the versions of iCal the same on each platform (or the latest)?
If you subscribe to calendars:
I've found that certain subscribed calendars don't work on all versions - as a matter of fact many of the public ones downright suck. So I'd turn off a few and test by process of elimination, assuming you've checked that each calendar subscription meets the minimum requirements for each iCal.
iCal isn't perfect - I've edited entries and suddenly all of them are locked out, then I simply quit/load and all is well again.
-jim