Docker Image Insecurity
I wish the author had not omitted this crucial paragraph in the announcement he quotes:    Note that this feature is still work in progress:
    for now, if an official image is corrupted or tampered with,
    Docker will issue a warning but will not prevent it from
    running. And non-official images are not verified either.
    This will change in future versions as we harden the code
    and iron out the inevitable usability quirks. Until then,
    please don’t rely on this feature for serious security, just yet.

So, we've made it pretty clear from the start that we're working on ways to make image distribution more secure, but are not claiming that it's more secure yet.