This script will display the last post or bmail in a thread while you are replying.
This was suggested here, however with Ender working on more important things atm, it may be a few months away before it is implemented. Until then, this should suffice for most users.
The script works by replacing the reply buttons in a bmail or forum thread with a javascript function, which saves the last post/bmail in the thread to your browsers local storage, before heading over to where you compose your reply. Once there it includes this information above your reply. http://i.imgur.com/9cd97uE.png
Grab the script from here: http://userscripts.org/scripts/show/175930