Whether it exists yet or not, I'm certain it will at some point. It's such a huge part of the game, really, that tongue-in-cheek ongoing gag, there is no cow level, but there absolutely is. I think the game devs want people to play all the other cool stuff in 4 first, before they introduce something that will inevitably tear people's attention directly away from it.

I think the same things happened in Diablo 3, there's no cow level until it's specifically added in some patch (the zone is called "Not The Cow Level" btw)

I want that. But I also want the rainbow goblins. I haven't played much yet so it's entirely possible they exist.

I wonder if they'll be able to wish it into existence.

It's fun to see this still ongoing. Back in the usenet days of yore I remember joining in on spreading the rumour about a cow level in Diablo on the alt.games.diablo channel. It was great to see the nods in both Hellfire and future Diablo games.