Does it work on another account...?
If so, maybe one of the calendar files is corrupt. Move all the calendars out of ~/Library/Preferences/Calendars/ (I think it was under Panther), then try to start iCal. If it starts, quit and then move one and one calendar back until iCal won't start. There you have the broken calendar file...
Edit: Too slow... nice to hear it worked... 🙂