So while this certainly counts as a World First, it doesn't look like Kael'thas is truly dead.
You can check out images below, and be sure to head to Nihilum's website to find out more on the World First. Loot listed below; the necklaces are from the Verdant Sphere quest. He also dropped a vial for every person in the raid that needed it.
Forte just killed Kael'thas as well; their loot is listed below.
With their freshly tuned characters, Nihilum went into Hyjal and killed the first boss of the instance, Rage Winterchill as a World First.
A bit after, Forte also killed Rage Winterchill(on their first attempt, and with only 23 people).