WordPress Posterous Importer Plugin
The current one I found did not work for me - perhaps too many posts. So I forked the code from http://wordpress.org/extend/plugins/posterous-importer/developers/ to create a plugin that would work.
I used some details from this forum post to fix the 403 error http://wordpress.org/support/topic/plugin-posterous-importer-403-error-message-when-running.
I used a Gist from Brad Gessler to fix a commonly reported link issue. https://gist.github.com/bradgessler/3185320