Can I configure OS X so that the default open action on an ICS files is to open it in iCal?
If you highlight the file in the finder, press command-i to open the file info dialog. Change the entry "Open with" to "iCal" (select "other" and browse for iCal if it is not listed). Press the button "change all" to make "open in iCal" the default action.
Take a look at this Apple StackExchange question and answer.