Deckset is a mac application for rendering markdown as presentation. It has a nifty feature of a preview window that can float on top of things. This window can be scripted using AppleScript, and this plugin will change the contents of the preview window as you move around in your presentation md file.
This is by no means complete. It is currently enabled for all markdown files, and the osascript call happens for all files, so depending on the speed of that it might anoy you. It doesn't anoy me yet.