The wordpress developers call it distraction-free writing or zen mode.
It’s what a couple of simple plain text editors have done on the desktop for years. Besides a couple of important features, this is the
new cool thing in WordPress 3.2 codename Gershwin. So for everybody who wonders what it looks like, here it goes:
Zen writing with interface on top which appears when the mouse is moved to the top.
Zen writing with no interface, just for typing.