Apple's mail.app stores most of the text strings it uses for creating replies, displaying dialogs, and composing new messages in an easy-to-edit text file. If you haven't relocated your mail.app, you can find the text file here:
/Applications/Mail.app/Contents/Resources/English.lproj/Compose.stringsWhen I tried to edit this in the terminal, all I got was gibberish - it's a Unicode text file. Opening and saving it in the GUI works fine; you just have to use a control-click on mail.app to "Show Package Contents" in a new Finder window, and specify which application to open it with (I used TextEdit).
man strftimeHave fun customizing, but you should probably make a backup copy of the unmodified file first -- just in case! Any changes made take effect on the next launch of mail.app.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20010401105726652