I just visited http://www.postgresql.org/download/linux/debian/ and my impression is that the way the signing key is published is not secured against wrong origin or manipulation by a man in the middle (MitM) attacker.
Meaning, that if a MitM attacker can compromise downloads, he or she is also able to compromise the documentation site including the source of the signing key, e.g. by publishing the attacker's signing key to the user. Debian's apt-get will not complain if everything fits together.
Therefore, I suggest that the whole page should be TLS secured (HTTPS-only), not because of encryption but to ensure origin and integrity of the signing key.
Work is under way to make the entire website available under https only. It's blocked behind some other work at this point, but once we get there, it should make this situation a lot better.
