There is a false assumption that IPv6 will make mass scanning like this impossible. In reality you just need to be more clever about it. (Remember way back when people used "needle in a haystack" security for dial-up systems, because nobody would ever have the resources to call every phone number in an area code?)

Link-local multicast (the replacement for ARP) allows tools like alive6 to very easily enumerate all live v6 addresses on a network. So once a spear phishing attack is sucessful, you can still scan the entire internal network.

Google hacks like "site:ipv6.*" and passive DNS monitoring allow you to easily separate used vs allocated/announced subnets on remote networks. IPv6 breaks in strange ways when you firewall ICMPv6, so ping scanning a subnet has become much easier.

There was also a great talk (i'll try to dig it up) that talked about predictable patterns in DHCPv6 implementations, so you can cut down v6 to a near v4 search space.

The best part of all is that very few security products on the market really support IPv6 correctly, so I suspect we will see more advanced attacks being possible because of IPv6 in the coming years than things being stopped.