I think they got drunk on their own popularity and are now feeling the pain of that ... potentially fatal mistake ... there's a lesson there for everyone.
I'd withhold judgement until the dust settles.
If this marketing campaign results in more active users for Posterous, from whatever source, then I think they would call it a success.
On the other hand, if they've alienated everyone and all the users leave then in hindsight, it'll just be considered a dumb move.
This whole 'Posterous vs everyone' reminds me of an Oscar Wilde quote -- The only thing worse than being talked about is not being talked about.