Really nice implementation, such a pity it's gone unmaintained

There are numerous Markdown plug-ins but I think this one takes the right approach in actually preserving the original Markdown content while at the same time not running the resource-heavy Markdown parser on every request. Mind you apparently the ‘post_content_filtered’ field can still get cleared up occasionally. There is a way to prevent it by adding a filter on ‘wp_insert_post_data()’.

Also, I don’t really see why to disable the rich editor. Fortunately, it is easily re-enabled with another filter. Ah, the power of WordPress filters…

Markdown Extra can be manually upgraded from 1.2.6 to 1.2.8 from Michel Fortin’s website (perhaps to a newer version too but they use a different interface and would require plugin code changes).