In Panther the ICS files which hold the calendars for iCal we're neatly stored in ~/Library/Calendars with meaningful names such as home.ics, and work.ics.
In Tiger these ICS files are now found in a file called corestorage.ics located in GUID-like folder names (e.g. 5FBF955B-4962-4A37-BA7B-9D23DA2231D3.calendar) under the ~/Library/Application Support/iCal/Sources directory.