Megaguy was the first person to reveal Adventure Mode to everyone. He wasn't the first to unlock it, but he is the first one who posted about it on the forums; I was there. Since the cat was out of the bag, everyone decided to just turn the topic into a Main Adventure Mode discussion. People would help each other and stuff. Then people started wondering how to unlock it, and someone else in the topic revealed you have to find the golden brick - or Mega mentioned that in the OP, I can't remember.
Anyways, since then, most people will discourage other players to find it by telling them it's fake. Towards the end of the topic people realized how much attention it attracted, and decided to start telling people it was fake. There are only a handful of people who have found it, I don't know exactly how many. Not a lot of people play it that much as before I think. Bisjac mentioned sometime that last time he checked only about 60 people were still playing it.
I don't blame people for telling others it's fake, but I don't see the point. It's very doubtful they will find the golden brick anyways to unlock the Main Adventure Mode. With my new key and everything I haven't even bothered looking for it again, pretty sure it's randomly generated. It's very hard to find and can take some time, to the point where it's not worth it. I guess you just have to get lucky.
Here is the original topic. For those of you who don't have it, this is probably the closest thing you will get to it for now and where you can get an idea of what it's like to play it. That is, until you find the golden brick and unlock Blockland Main Adventure Mode.