How to Change the Attribution Line in Mail 6.0?
After some searching, I am pleased to advise that I found the solution to my own question, as well as the solution to a few questions from others here:

There is a small utility app called Quotefix that is available here:

http://code.google.com/p/quotefixformac/

With Quotefix, you can change the attribution line to anything you'd like, and for those
asking about it, you can make the cursor appear below the quoted text in replies, and there are several other goodies that I haven't tried yet, as well. Kudos to the creators
of Quotefix for this -- it works as advertised and my attribution line is now exactly as I want it.