Some meeting invites sent from Outlook just do not work with iCal and when you click on the invite (.ics file) iCal reports "iCal can’t read this calendar file. No events have been added to your iCal calendar." I remember reading a hint (sorry, can't find it to reference it!) that pointed the blame at empty lines at the start and end of the .ics file. If you remove these (using BBedit, for example), the invite is recognized by iCal.
So after doing this too many times, I came up with an Automator solution to speed things up. It may not be the most elegant, but it does work. The following has you create a one-off directory and apply a Folder Action to it. If you save an ics file to this folder from Mail.app, the Folder Action will run a script to remove the empty lines, save the file, open the file (and therefore load it into iCal), and then remove it to tidy up.
Usage: From Mail.app, save the ics file to the FixiCal folder, and Automator does the rest. Read on for the how-to...
grep -v "^$" $@ > ~/Documents/FixiCal/fixed_invite.ics
on run {input, parameters}
do shell script "open ~/Documents/FixiCal/fixed_invite.ics"
return input
end runMac OS X Hints
http://hints.macworld.com/article.php?story=20091215080321140