I've done some checking and www.stackoverflow.com is currently hosted on an IPv4 address that belongs to peer1, so this must be PA space.
Looking on ARIN's whois, Peer1 (PER1) have lots of IPv6 space:
PEER1-IPV6-01 (NET6-2001-1978-1) 2001:1978:: - 2001:1978:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF
PEER1IX-IPV6-01 (NET6-2001-504-19-1) 2001:504:19:: - 2001:504:19:FFFF:FFFF:FFFF:FFFF:FFFF
So you can get an allocation from the 2001:1978:: space.
I've had a look at Peer1's IPv6 routing and it seems to be pretty much first-class, so there's no ISP excuses for not having IPv6.
I know that stackoverflow have private BGP (from the SF blog) so they should be able to just get an IPv6 allocation, and announce their routes through BGP.
The web servers (being Windows 2008) support IPv6 straight out of the box, so the constraints I can see are:
- Do they store / use IPv4 addresses anywhere, e.g. in spam-blocking? If so, they have a development job there to incorporate IPv6 addressing. Not that huge, but still some work.
- What about their load-balancers, routers, etc? - HAProxy and Quagga both support IPv6 in recent versions, and if they have switched from Quagga to Cisco hardware routers, then all recent versions of IOS support it too.
Conclusion: StackOverflow could operate on native IPv6 without too much of a hassle, and it would be nice to do so and will get steadily more and more important.
How about World IPv6 day (8 June 2011) as a target for IPv6 native throughout?