I wonder if Notch is regretting releasing these details so soon. Now he's already going to be a slave to backwards compatibility and the game is about 0.1% complete.
Seeing that everyone is doing this just for fun, I doubt it. He could change everything tomorrow and I bet people would be excited to do it all over again.
I don't think everyone's doing it for fun, I'm sure there are savvy dudes out there that know that there's a lot of money to be had by being a first mover in the Notch ecosystem.
But even those that are doing it for fun -- you might be underestimating the amount of nerd rage that people are capable of when stuff they don't want to have happen happens. Just sayin'.