He says "I have never tasted death, nor shall I. But that is a tale for another time" http://www.youtube.com/watch?v=ZRk_ZBQ0C2E daaaamn, I want the other campaigns
I was wondering that myself. And after the ending of WoL I can only assume that we won't even find out until the Protoss campaign surfaces, which isn't for about 3 and a half years....
I don't think he's Tassadar. Way to convenient. Guessing some kind of elaborate scheme by the "Fallen One"
I think at least the obvious conclusion is that the overmind was such a powerful psionic being and already had that whole phoenix gambit where it could regenerate if you killed it without using the void that tassadar had to find some way to turn into a void energy being that has to sit there maintaining the seal on the overmind so it can never respawn.