That said, Tumblr's features seems to be a subset of posterous these days. I am a big tumblr user, but I ended to migrating to Posterous for it's much less work for even greater sharing power.
The final straw was posting code. Tumblr makes that stupid easy (since they accept Markdown), so I can just do that instead of trying to figure out the magic incantation to make it work on Posterous (gists or something? I don't know. It wasn't documented when I last tried).
Markdown etc has been a much-wanted feature, even for ourselves internally. We try to focus on normals as much as possible, so it hasn't hit the top of our priority list yet, but it has been rising in the list without a doubt.