The End

The Ender is a dimension soon to be shown, mentioned by Notch. It is possible that this is the official name for the Sky Dimension, rather than a new dimension at all. This dimension will be home to the Endermen and Enderdragons.

History
Notch tweeted "raqreqentba", which could be decoded using ROT13, translating to "enderdragon." A Reddit user under the name of "cptqwashi" posted the idea that the new dimension accessed by the new "Crystal Block Portal" would be called the Ender, and would be home to the Endermen and Enderdragons. Notch soon gave more information and said that cptqwashi was "100% correct."

Trivia

 * The random Ender-Portals (Crystal Block Portal) in Strongholds may explain why there are Endermen outside of their native dimension.