This should probably be filed as a support ticket (too), but as there are people who write plugins etc here, it fits the forum.
Strait to the point: WordPress MU lacks IPv6 support. Or.. not really. It works. You can read, login, write, publish, comment, have fun, it all works with IPv6 as of now. But there are some minor issues.
For example, when I post a comment my IP is (supposedly):
2001618400012 (without any . or :), which - if you are aware that IPv6 exists and that the lack of . indicates IPv6, lets you sort of make out that this guy's using
2001:0618:0400:012/64 (which is only a subnet)
The truth of the matter is that my IP in reality is something like:
2001:0618:0400:0120:0000:0000:0000:0000
or 8x sets of four digits between 0 and f.
Of course, you can learn that HE'S ON THE SUBNET 2001:0618:0400:012/64!! but I could also be on 2001:0618:0400:012f/64 for all you know and you're a long way from figuring out that 2001:0618:0400:0120:0000:0000:0000:0000 is the IP in question.
So, something to keep in mind: There are IPv6 supported WPMU sites. Make a habbit of ensuring that things work with long :-seperated IPs too. As said, there are no REAL issues with using WPMU+IPv6, it works, you can signup, blog, have fun, nothing of any real importance breaks, but there are those issues like here's-only-half-of-the-comment-posters-IP.