Apple had a very, very busy on Monday, June 11. With multiple MacBook Air and MacBook Pro, as well, as Airport and Mac Pro, hardware upgrades out in the wild, software updates were flying fast and free — these specific patches bring improved IPv6 compatibility and support for the new Airport Express (Apple Store) base station.
AirPort Utility 6.1 for Mac, download
— Allows you to configure the base stations for IPv6 and includes additional bug fixes.
AirPort Utility 5.6.1 for Mac, download
— Allows you to configure and manage the new AirPort Express with simultaneous dual band 802.11n.
— Also available for for Windows
For what it’s worth, the final transition from IPv4 (not enough IP numbers) to IPv6 (plenty for the foreseeable future) happened recently and Apple’s tweaking their network gear to insure smooth operation.