Hmm, have the height limitations been removed? I know at one point Minecraft worlds were technically infinite horizontally but limited to...256? 512? meters vertically, from bedrock to skybox, which is obviously not enough for most real-world terrain. I haven't played in ages, though.

I saw a video from one modder who managed to, not remove, but rotate the height limitation, producing a cavern world that was 256 meters wide and infinitely long and deep. He tweaked the terrain generator to accomodate it, too; there was no surface, just uncountable miles of vast tunnels. It was creepy.