WPCentral posted a news story that Windows Phone 7 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. 

  WP7 doesn't support IPv6 yet