This is a clear example of "good enough." Low security for low value targets -- if you need more you can get it. Setting a password, remembering a special email address, not posting via blackberry/mobile, all of these add friction. EDIT: Although it is fun to think of solutions ... Posterous could mail you back a link; when you hit the link the post goes live. Then you would clearly need control of the sending addres…
This is going to be a serious issue for Posterous if they ever go mainstream. Opt-in authentication schemes won't be enough to prevent scores of naive people from being humiliated the first time, particularly teenagers.