It can be actively exploited in the same way that TCP initial sequence numbers can be exploited: when something else goes horribly wrong in the protocol stack. Contra the claim across the thread that the fundamental fix to this problem is DNSSEC (which is never going to happen), the real fix for all this stuff is not to trust this layer of the TCP/IP stack for authentication in the first place: you do cryptography, at the transport layer (not in the name lookup) so that IP address spoofing doesn't matter in the first place.