So now that we all know that Gearbox is finishing this game, one thing I'd like to know is, what engine is DNF utilizing in it's current state?

Quake engine V12.4547

Unreal Engine 2.5?

I assumed it was probably Unreal 3(considering Gearbox's recent games have ran using it. I.E  Brothers in Arms: Hell's Highway, Borderlands, upcoming Alien game), but I really don't know.

It is Unreal 2 heavily modified to hell.

Its actually the original quake engine not modified.

Probably Unreal Engine because that's what Gearbox uses.

Gearbox is not porting it to a new engine. They are polishing what 3D Realms has done. So unless 3D Realms did it on Unreal 3 engine, this game will not use it.
 
I have a feeling it's using id tech 4 (Doom 3, Quake 4, Call of Duty 2+).

I believe that the last time they confirmed what engine it was running on was in 2004 and it was running on id Tech 4. That's still a viable engine today, so they could still be using that. But it's not a popular engine, and gearbox have no experience with it, I wouldn't be surprised to see it running on Unreal 3 now.

Well either way it still looks good, even from the shitty live stream thing you could tell that it has better lighting effects than most FPSs today.

According to Wikipedia, in 2004 they switched to id Tech 4 as the engine. So they probably used that up until they closed doors, and that's what I'd bet Gearbox is using to push it out the door.
 
Porting to a new engine seems like more work than it's worth.

They posted this in late 2008:  
 

 

" According to Wikipedia, in 2004 they switched to id Tech 4 as the engine. So they probably used that up until they closed doors, and that's what I'd bet Gearbox is using to push it out the door.  Porting to a new engine seems like more work than it's worth. "