This is an excellent-ish idea. I'm not a big fan of pay-to-win games however, if there was an in-game store for in-game currency, I think it is an excellent idea.
Any popular MMO eventually draws in real money for in-game money purchases, so why not just make it part of the game (and allow for both) and use part of the monetary purchases to lower costs for everyone?
Notch is effectively creating an in-game "craft" that actually requires skill. And just like in the real world, people who have that skill should be allowed to profit from it.