I watched him coding some of the engine the other night on his streaming feed and these screenshots shown here are through various stages of development when testing theories and different methods. I saw him go from having static blocks (a la Minecraft), to having editable blocks that you alter in real time yourself like a basic 3D modelling program, and that you see here in these screenshots is the stages of this.
These came from twitter where he was messing around and prototyping different things for a laugh such as adding the wood texture from minecraft to the polygons he has in his prototype engine.
I dont agree with everything Notch does from a business point of view, such as how slow and incomplete minecraft is with a shed load of bugs and stuff, but I feel I ought to at least set things straight with the comments in this thread that these screenshots are from the "finished" game. They clearly are not. They are from the first few stages of development of his level designer. Moaning about how crap it looks this early is as stupid as moaning how bad Quake 3 would have looked before any texturing and animation and even the rounded vertices used for archways and stuff. Its work in progress.