So, thanks to a comment from Richard, I discovered on saturday that my little posterous backup script did not backup private posts.
I initially found it weird since I knew that, in my script, I correctly set the basic authentication mechanism.
In fact, it was due to the way posterous api is built, I think. From what I've understood, there is no session maintained on server side (weird choice, Sachin).
So, from Tom's advice, I updated my script to ensure the basic authentication token where written on all requests.
Now, Richard, posterous export should grab all your entries, even private ones !
Besides, I've also copied that script on git (since relying solely upon dropbox survival does not seems enough to me). So, if you want to update it, it must be rather easy, no ?