Well wonder no more. A concept art of what Big Daddy's look like in BioShock was tweeted out and it's as horrible as you would think. While you got to play as a Big Daddy in BioShock 2, you were never treated to what they looked like underneath the helmet. And in the second game, you were prototype to the ones that wander Rapture.
I have to say I was much happier not knowing what monstrosity was under that helmet. It doesn't look like a happy individual and I would hate to see its face when it gets angry. Maybe with the new BioShock game in the works, we'll get to see them in the flesh besides this concept art.