To me, a Markdown preview is somewhat orthogonal to the process of storing and retrieving notes for personal reference, so I've been hesitant to add it so far. But if people derive value from a live Markdown-processed web page, then I can try to find a way to incorporate it without it interfering with the normal workflow.

But I would appreciate it if you could give credit by linking to the actual app from your post:

http://notational.net/