Scrivener has been available on Mac for about for 5 years now, I think. Based on the original website, it seems to have a bunch of nice writing features.
They are just now coming out with a Windows version, which is done by a different programmer. Although the public version won't be available until early 2011, a beta version will be made available for NaNo sometime in February.
They are still field testing, hoping to get some bug reports, so it may not be a good choice for everyone. But I've read lots of good things about the program, and the final version will be available for purchase during NaNoWriMo at half price.
That makes it something I've at least gotta try.