Use your API Key to connect to MailChimp, choose a list, enter in your Posterous credentials and Chimpsterous does the rest.
Your Posterous will now contain a new page titled "Newsletter". You can always rename or delete the page by logging into your Posterous admin.
Great idea in theory, just isn't working (as of 29/03/12)
Sorry, couldn't find where to ask for support. When I tried to sign up (awesome idea, just what we needed) after the last step I got a big white screen of death with a little bitty error message "Sorry! Something went wrong. Cannot convert nil to string." I'm paraphrasing a little.