It looks like Microsoft Windows Server 2008 SP2 changes the way IPv6 works. Perhaps others can corroborate my experiences, or tell me I’m nuts.
I’ve been running SP1 with a fully-configured IPv6 stack for some time. I installed SP2 today and two things happened:
1. It appears that the Teredo tunnel now takes precedence over an actual, working IPv6 stack. I was unable to connect via IPv6 to local resources and “ping” returned insanely long response times (300 ms vs. the 1 ms it should have been).
To disable this from the command prompt I issued the command:
netsh interface teredo set state disabled
This smells like a bug to me, and at the very least it’s annoying if you have IPv6 connectivity.
2. It appears that the IPv6 privacy settings get reset, so if you have a static IP you won’t end up using it, but instead will use a rotating IPv6 IP. To disable this from the command prompt:
netsh interface ipv6 set privacy state=disable
You’ll have to restart after setting this (at least I did).
I’m a Linux & UNIX guy, and really don’t count myself as a Windows expert, as I know just enough to be dangerous. :-) If someone can add to this (and not just MS bashing) please leave a comment.
 Every OS vendor has situations like this where a patch changes the behavior of a component. In my experiences Microsoft isn’t even close to being the worst offender.