This is a simple WordPress plugin I made to automatically activate the fullscreen writing mode when creating a new post or page. It stems from an idea Patrick Rhone mentioned on app.net.
Editing existing content will still show the regular editor with all options visible. For quickly writing new content, though, this will give you just want you need: a clean, simple writing area.
The code is hosted on GitHub. It has been tested in WordPress 3.5 and if you find any bugs please open an issue on GitHub.