Sky dimension

The Sky Dimension (otherwise known as the Floating Island Dimension, sometimes referred to as heaven) is a work in progress by Notch. It "performs well, [but] looks ugly". It is now a confirmed update, although it probably won't be implemented in 1.6.

The Sky Dimension was first announced after Notch played Spleef against @Evil_Notch (video is on twitter). It was probably made to be like a "heaven", since the "Evil Mojang" team declared the Nether to be "hell". The sky dimension was not completely confirmed until Notch released an update via Twitter stating: "And because PC Gamer made such a big deal of the sky dimension, I guess I'll have to finish it. Consider it confirmed."

On the Minecraft Forums, the Sky Dimension is called the Aether. It is made to be the opposite of The Nether. It is a collection of many ideas for new creatures and blocks, but not all of these may be possible because of the size of the block texture file.

Other info

 * It is currently unknown whether or not the "Sky Dimension" will have any of its own items or mobs, although the pictures at this point seem to suggest that it will not.