WordPress have announced that they have created a Posterous Importer for hosted customers. Like other importer tools, this takes the content on a Posterous blog and imports it into a wordpress.com hosted blog.
The Posterous importer can import posts, tags, comments, and image attachments.
It uses a Posterous API for reading from and writing to their service.
More Info: http://en.blog.wordpress.com/2010/02/09/posterous-importer/