"We will need this in 10 years, so we should start learning about it now" is an argument, I guess, but most ISPs of my acquaintance are more concerned with staying business for the next five years than they are with optimising their costs in ten.
HTTP, IM and P2P all showed a doubling of bytes transfered over the protocol every 2 weeks for at least 6 months. In the case of HTTP and P2P they became the dominant amount of traffic for some networks inside that 6 months. *If* IPv6 ever grows a killer app (big if...) an ISP may discover that it's traffic is all inside IPv6 tunnels, and their traffic engineering no longer works. An Enterprise may discover they are unable to firewall inside IPv6 traffic. Everyone may discover that their traffic monitoring software no longer reports useful results (Why is 60% of my traffic protocol 41?). "We'll just firewall all IPv6" just makes everyone's Internet slow. You might have less than 6 months to deal with this. IPv6 is dissimilar enough from v4 to require some retraining. (What happened to all the ARP messages? Why don't I need DHCP? Why *do* I need DHCP? Why are addresses that begin with Fe80: special? 2002:? 3ffe:? What's the deal with RFC1918? Is it normal my machine has nearly 100 IPv6 addresses on a single interface? Why doesn't doing the obvious ping of a link local address work? Why shouldn't I assign ...:feed:cafe:babe:f00d as an address to a machine? Given a mac address, what IPv6 address would be dynamically assigned to this host? What's the story with DAD? and what's he doing on my network? I have an MTU of 576 on a link, what happened to my v6 traffic? Would it be better if it was 1000, 1100, 1200, 1300, 1400, 1500? How am I going to remember all these v6 addresses? What's the v6 address for localhost? What's the broadcast address on a v6 network? How do I enable v6 on the various host types inside my network? More importantly, quick, how do I turn it off again? If I have an IPv6 address on an interface, does that mean I can talk to v6 hosts on the Internet? Why does connecting to some hosts on the internet now take a long time, it didn't yesterday! What changed? My network is v4 only and has a NAT box in front of it to protect me from malicious traffic, how come all of these machines can talk to v6 machines on the internet? How come v6 machines on the internet are successfully talking to them! Whats the story with Site locals? Where's all this multicast traffic come from? What's the HD ratio, and is it important to me? How do I multihome? What's a home agent good for and why does my machine want one? Who's IKE and what's he doing in my kernel? What's the v6 equivilent tool for <x>? How do IPv6 only hosts talk to a IPv4 only host? How do IPv4 only hosts talk to IPv6 only hosts? Someone once said IPv6 gave me {security, QoS, addresses} for free, are they lying? misinformed? or is there some tiny element of truth? What are the pitfalls with v6 addresses and DNS? Do I need to update my resolvers? What's ULA, do I need it? Should I give dialup users a /20, /32, /48, /56, /64, /120, /127, or /128? What about DSL users? What about colo'd users? What about my fridge? Given an IPv6 address, can you derive it's MAC address? What are the privacy implications of this? How are these addressed? What's 6to4? ISATAP? 6over4? Teredo? SHIM6? Are these acronyms going to haunt my nightmares? How do I do NAT in an IPv6 world? What's AH and why is that going to mean I can't screw with my users traffic anymore? What's ESP and is that going to make firewalling troublesome? And what's the story with QoS? How on earth am I going to find 2**108 customers in two years to keep APNIC happy?)