The Highborne are planning to return to the Kaldorei society at large. This is most likely how Blizz explains the soon-to-be-available night elf mage class. This is yet another change when many people cry out and yell "lorelolzomg", but I do prefer an evolving Azeroth. (Role)Playing in a static world that is mostly based on Warcraft 3 for 5+ years tends to become boring after a while. So hooray for the winds of change!

(Of course there are still many things in WoW that don't make much sense though)

You can find Mordent Evenshade in the Temple of the Moon, Darnassus.

http://www.wowwiki.com/Highborne

http://www.wowwiki.com/Eldre'Thalas

Check out www.youtube.com/user/Melvenor for more.