Trying to import Mozilla calendar entries into iCal should be a no-brainer. But although they use the same format, the import of the exported Mozilla file does not work. It just imports the first entry. This seems to be due to the presence of:
END:VCALENDER
BEGIN: VCALENDER
between each entry. I erased them and placed my export file in the ~/Library -> Calender directory, restarted iCal, and voila -- it worked! So if you have problems with importing something, check the files. They do not have to have big differences to cause an import failure.