WP7 doesn't support IPv6 yet
We've had IPv6 support in Windows for years, but Windows Phone 7 seems to be lacking the successor of IPv4 (one of the Mango features?). v6 has been in talks for a while and Google and other big websites adopting the protocol with network administrators and testers preparing their infrastructure accordingly.
While it's not actually needed at the moment (since networks and ISPs still support IPv4), there will be a point where hardware such as routers will need to be updated (or replaced to increase profit for vendors). With your device, we should see Microsoft adding support for the protocol when networks and carriers start to look at making the change.
Could Nokia be bringing an IPv6 push to WP7 when their devices hit the shelves? Could well be, but for now consumers shouldn't really be thinking about IPv6 since we have a long way to go until the change over from v4 is full steam ahead.
Update: See this post for follow up information
Via: Smartphone France