Merlin reviews Scrivener, a new Mac OS X app for writers. The Scrivener home page. Looks awesome. $34.99, with a 30 day free trial.
Re: full screen mode, see also Mark's recent post. Re: full screen mode writing and Mac OS X, I'll paraphrase a tip from Merlin: Use Backdrop and MenuShade to put a solid background behind your favorite editor and hide your menu bar, respectively. Editors are terribly complex, and it's terribly important that you learn how to use yours well, so you shouldn't have to switch to an entirely different editor just for one feature. Better to assemble the necessary features from different pieces of software.
But Scrivener's full-screen mode is more than just a black backdrop behind your editing frame, and of course the editor looks worth considering for its other features.