Welcome to WordPress stack exchange! Just yesterday I created WP-Markdown, which enables Markdown on your posts.
It also provides the same interface as this website (the toolbar, and live preview) – however, currently only for comments and bbPress forums (if enabled). That said, (since it would be straightforward to do), I am planning on adding an option to enable a ‘back-end’ live preview. Hope that helps.
Edit: Since 1.1 this now includes an option to enable the toolbar and preview for the post editor, and so now actually does what the OP asked for :D.