I wanted to shorten the long warning message that appears when starting a chat in Microsoft's Messenger for the Mac.
After some digging, I found the solution. Navigate into the Messenger application (control-click on Messenger, and select Show Package Contents). Then navigate into Contents -> Resources -> English.lproj. There you'll find a file named InstantMessageWindow.strings. Open it in your favorite text editor.
Now change (or make it empty) the string definition for IMWarning, which you'll find at the end of the file:
/* IM warning message */
IMWarning = "Never give out your password or credit card number
in an instant message conversation. To help prevent infection by a
computer virus or worm, never accept or open any file or link in
an instant message until you verify its authenticity with the sender.";
Change the string to whatever you want, save the file, and then launch Messenger again.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060224033358785