Apparently, based on a screenshot of Kael’Thas in the Sunwell Plateau, Kael was resurrected by Kil’jaeden and transformed into Undead. He no longer looks like a normal Blood Elf. His complexion looks undead and skinny. A green crystal is embeded into his chest. Nothing official, just comments based on observation.
Previous major content patches have been accompanied by a Lore page. Hopefully there will be one for patch 2.4 shedding some light on Kael’s current condition. Keep an eye on the official WoW Site.
The PTR Servers and Character Copy in US are still down. I don’t post images other people have extracted from the MPQ. Read the previous newspost for constant 2.4 PTR updates.