Read through the comments on that page.
I'm hoping Notch goes with
Tesseract as the name. Name of four dimensional cube as the name of the fourth dimension, it seems rather fitting and still sounds bitchin' cool.
I suggested calling it 'The Mines' implying that it's not a fourth dimension but tunnels much closer to the core of the world (less surface area = distance multiplier) left by ancient evils/herobrine/loving creepers, but Tesseract kicks the stuff out of that suggestion.