To resolve the issue, update your AirPort Extreme firmware to version 5.5 or later, using the update found here. If you are unable to connect to the base station to update the firmware, perform a hard reset.
If that doesn't work, try disconnecting the base station from power, plugging it back in, and then using the "AirPort Setup Assistant for Graphite and Snow" utility, which is located in your Utilities folder next to the default version of AirPort Setup Assistant.
This document will be updated as more information becomes available.