I know I’m late to the game but I’m now using John Gruber’s [Markdown](http://daringfireball.net/markdown) for writing on this site as well as documentation at work. I’m thoroughly enjoying it.
Oh, I’m using [Mark Jaquith’s](http://markjaquith.com/) [Markdown on Save WordPress plugin](http://wordpress.org/extend/plugins/markdown-on-save/) and it works great.