The size of the IP6 address space makes scanning for victim computers in a properly managed network as difficult as a brute force attack on an encryption system. Of course, it also makes scanning your own network for backdoors and trojans just as difficult. Furthermore, crackers are using IPv6 to encapsulate traffic, hiding it from intrusion detection systems.

This is a fun paper if you are into network security.

Security Implications of IPv6 - Mike Warfield