Out of curiosity, what would a "visual MD editor" look like to you, and how would it be different from a plain text field or a split screen like http://attacklab.net/showdown/ ?
if you missed it, you might be interested in http://wiki.pageforest.com - posted to HN last week. It does live preview of markdown text in the browser.
Anyone want to tackle a bookmarklet version to inject ReMarkdown into any page? Even better, a Chrome extension that Mardownifies thebwhole Internet for you...