On Emacs 24.4, `(toggle-frame-maximized)` toggles the maximized state of the current frame.

See also, the [Maximize Emacs on start up](https://stackoverflow.com/questions/7763847/maximize-emacs-on-start-up-not-the-fullscreen) question on stackoverflow and the [Full Screen](http://www.emacswiki.org/emacs/FullScreen) page on the EmacsWiki.