This is an interesting case. Started by Garry Tan as a consequence of the somewhat characteristic decline of Posterous, it seems like they are aiming to only slightly update the Posterous model on a technical level—post-by-email, simple frictionless blogging—instead solving the problem of longevity and and service survival. That is, addressing the fact that this type of service, which is almost always free or freemium, has a tendency to go away, either because the service has been bought out, or because the service has not managed to be bought out.
Posthaven is charging $5/mo from the very outset, with the pledge that they will effectively never shut down, and that anything published with them will remain accessible forever.