If iCal gives the error “iCal can’t read this calendar file. No events have been added to your iCal calendar.” and you’re trying to import an ics file, try using TextWrangler (or some other method) to convert the line breaks to Unix (LF). This is near the text encoding value (e.g. UTF-8) and also present in the Save dialog box. I ran into this error when trying to import to iCal 5.0.3 from Thunderbird 12.0.1.
Neither Windows (CRLF) nor classic Mac (CR) line breaks worked, the former was the default as exported by Thunderbird.