[d0nk]

Other people replied about a randomized/hard to guess email address. Building on your idea of a "hard to guess word" or token in the subject, posterous could allow a user to upload their public key, and require that their posts be gpg/pgp signed with their key (obviously this would be something that users would have to opt into). The displayed post could simply strip out the signature as it would only be needed for authentication.

[Tichy]

Only with that kind of thing, or even the Zucchini method mentioned above, they would kind of lose their tag line about the easiest way to update a blog (or whatever it is).