A recent article in comp.sys.mac.comm talks about how to change the attribution line in Mail.app (using OS X 10.2.x).
- Edit /System/Library/Frameworks/Message.framework/Resources/English.lproj/Message.strings in TextEdit.
- Change REPLY_MESSAGE_PREFIX and REPLY_MESSAGE_24HOUR_PREFIX to whatver
you want, such as "%%@ said:\n\n" - Save your changes
Don’t forget to make a copy of the original file before you start editing things.