Trust, but verify

Actually, it has. There was a neat trick way back when SSH servers still accepted both protocol 1 and 2. A man-in-the-middle could force a change in protocol by changing the packets so that it was probable that the host key would not be in the client's known_hosts file. The user would receive a relatively benign 'The authenticity of host X can't be established' message instead of the hostile 'WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED!'. It would be easy for an inattentive user to ignore the warning and accept the compromised connection.

More details (although with awkward text flow) at: http://hubpages.com/hub/sshprotocol