So apparently Mojang decided to relase a F2P version of Minecraft in the future.
The new plans are that free users can use only use weapons and tools up to iron and have to pay 0.25$ to mine a diamond. They aswell can't make boss fights or go into the nether.

Watch Notch talk about it here: https://www.youtube.com/watch?v=oT3mCybbhf0