There's a big difference between "Um... ah... we'll tell you before we delete everything" and "We've received assurances from Twitter that they intend to support the project for the foreseeable future", though.

Yes, but if they haven't received those assurances, there's no benefit in highlighting the fact (for either the users or Posterous), and much potential harm if users spook and Twitter later decides to continue supporting it.

They would have just been through a negotiation process for acquisition. Of they didn't seek out assurances or none were given I find it highly unlikely there will be any result but a shutdown. If twitter was interested in posterous the product, it would have been front and center in the announcement.