I used to be good friends with a Blizzard employee, which is who I heard the rumor about fighting Tyrael from. Supposedly he somehow becomes corrupted and joins Diablo's side.
And yes, Diablo is back again. Though Blizzard is unsure on whether or not they're going to bring back Mephisto and Baal.
Though, all this reminds me, I haven't talked to this guy in some time now. I really should call him up and check up on D3. =P