Boubouille from MMO-Champion has recently posted this "leaked" screenshot of Kil'jaeden in-game. According to the sender, this shot of Sunwell Plateau's final boss was leaked from a GM.
"Keep in mind that it could be a fake, it's not "that" hard to get the 2.4 running on an emulated server and spawn Kil'jaeden, in this case his scale on live servers could be different," warns Boubouille.
Kil'jaeden the Deceiver, as we all know, is the eredar Supreme Commander of the Burning Legion, perhaps only second to Sargeras in the badass pecking order. With his appearance in-game and a chance for 25 intrepid mortals to "take him down", the lore implications are interesting. Granted, we only get to deal with "half" of him (the rest of him is submerged in the Sunwell), but lore-inclined raiders will certainly be itching to take a shot at him, at least until Arthas/Ner'Zhul/the Lich King makes an appearance in WoTLK.
I'm not certain of the authenticity of the screenshot myself, but Killie definitely looks constipated and evil enough here to be the real deal.