It's something that most forum software has. Why the phpBB team didn't include it is beyond me, but then again they didn't include quick-reply or quick-edit, either... and those are relatively old inventions.
Anyways. Would it possible to install this mod, or for the Powers That Be to code their own?
http://www.phpbb.com/community/viewtopi ... 0&t=544839
It essentially just puts the title of the last post in the "Last Post" column of the forum index.