Especially directed at PettingZOOPONY:
As a programmer myself, this sounds like a good decision. Look at the situation now - the code is heavily obfuscated, with each patch, you'll have to remap what is what. Yes, you can decompile the code, but it still takes an effort. Now, with Notch opening up the source (even with some heavy license agreements attached to it), you'll get a better interface with the game, are always on top of the time, will always target the same names in source, making your work much smoother.
Of course, it depends on the license in the end if this is worth it - with just some tidbits published, you can't really tell. However, I don't think they will be that bad, it is Notch's way of getting somebody to court IF shit hits the fan. And this is his full right to do, protecting his IP (yes, even if it's based on Infiniminer, there are vast differences). He knows that the eyes of the world are upon him with his recent success and people will cry doom at everything.
Personally, I wouldn't even have much of a problem with a fee for the access - simply because it keeps people away who just want to make problems. Yes, this whole development might mean, that most of Mojang will be busy with other stuff and cherrypicking mods to buy and implement when they fit the idea of minecraft. To everybody willing to have a go at it: Read the license carefully. While I don't think this will end up in a dick move, it very well could.
tl;dr: The license is Notch's way to have the power to go to court if somebody makes money of the code without having the license to do it.
