One thing that irks me about the iCal.app / Mail.app integration on OS X is that iCal chooses (based on some unknown criteria and without user input) the sending address (and hence account) from which to email invitations and responses to invitations.
Since I have multiple calendars and multiple email accounts, it is often that I want to choose the specific account when sending an invitation or replying to an invitation. This 40KB tar file contains the iCal AppleScript changes necessary to create an account selection box prior to sending these emails. Once a sending address is selected and OK is clicked, Mail.app will proceed with sending the iCal invitation, or replying from the account corresponding to the chosen email address. Here's how to install the modified code.
First, quit iCal and create a backup. You can do that however you wish; here's a version that uses Terminal to first copy iCal, then create a disk image of the backup:
$ cd /Applications
$ tar xvf ~/Desktop/LeopardiCalSelectSender-20071127.tar
Run iCal.app, send an invitation and reply to one, and test out the ability to select the sender for the outgoing Mail.app email. Note that you can read the original version of this hint in this post on my blog.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20071127201916589