This post is obsolete since WordPress now allows to write posts with Markdown directly (in the dashboard: Settings / Writing / Check the Markdown box).
I’m leaving it for the record and MarkdownPad is still a nice tool.
I’m getting a little tired of composing my posts in HTML so I’ve downloaded MarkdownPad which enables to convert from Markdown to HTML.
Once the text is finalised in MarkdownPad, Tools > Preview Markdown in Browser (F6) then right click > view source in your favourite browser, copy paste the body to WordPress.com et voila.
The only caveat is that code is formatted as html code but looks better when using the
[sourcecode] tag which adds nice-looking syntax highlighting. This has to be done manually.