Don't like the text of the e-mail iCal sends out as an invitation? No problem, it's easy to change!
Show the package contents of iCal (control-click on the app and select Show Package Contents), navigate to Contents -> Ressources -> English.lproj (or any other language) and open the file iTIP.strings in your favorite text editor.
Here you can change all invitation email texts and headers as you wish. The comments in the file nicely show what you can change, and you can also change the subject line. Just be sure to only change the part behind the "=" sign of the text you wish to change, and don't forget to save! ;)
Now restart iCal and all your invitations can have the text you want.
Mac OS X Hints