public function validateHost($host = null)
    {
        if ($host === null) {
            $host = $this->_host;
        }

        /** 
         * If the host is empty, then it is considered invalid
         */
        if (strlen($host) == 0) {
            return false;
        }

        /**
         * Check the host against the allowed values; delegated to Zend_Filter.
         */
        $validate = new Zend_Validate_Hostname(Zend_Validate_Hostname::ALLOW_ALL);
        return $validate->isValid($host);
    }

I see two problems here:

1) Zend_Uri_Http::validateHost currently defaults to ALLOW_ALL when validating the hostname. This is inconsistent with other sections of code (Zend_Validate_EmailAddress, Zend_Validate_Hostname) which default to ALLOW_DNS (the expected behavior based on the documentation for Zend_Validate_Hostname).

2) There is currently no way to override this flag in the validateHost function.