I found this blog today as I was looking for more information on writing with a Markdown Editor: Journal - live your best dreams. Really great stuff.
I downloaded Byword • A Simple Text Editor for Mac and already love it after about 10 minutes. The shading of text really is nice (Byword shades the Markdown stylings in a light shade of gray).
I was having trouble getting Brett Terpstra's Scripts to work (I'm positive it's user error on my part) but I found The Macolyte's script and it worked for me. I used QuicKeys to program a key command and voila, when I have a file I want to preview in Marked, a couple taps of the keys and I am there.