Apple Mail - Making Reply and Forward Attributions the SAME?
I'd be very interested in this if you can pull it off, as that reply header format is super annoying to me.

For the previous version of Mail, a guy at Purdue wrote a plug-in to do exactly what we're looking for here.  He's since moved off OS X so he isn't maintaining the code for Lion, but it might serve as a good starting point for you.  The source is at:
https://github.com/saptarshiguha/ReplyWithHeaders

The author gave me the following tip (all Greek to me, but might be helpful to you):


  Fixing this to work in Lion might be as simple as including new UUIDs
  (plugins require something called UUID compatibility IDs), or a
  rewrite (the Mail internals might have changed).


Hope this helps!  I know a lot of people that would be very excited to fix this little shortcoming in Lion Mail.

-Rob