Apple has released an update for called AirPort Software 3.2. Though the update is titled simply "AirPort Software," and not "AirPort Extreme Software," the update notes say that the update is recommended for AirPort Extreme computers and base stations. The software includes new wireless security enhancements, and other improvements. From the description in Software Update Control Panel:
This software update provides improved AirPort wireless networking software, and is recommended for all users with an AirPort Extreme enabled computer or an AirPort Extreme base station.
New AirPort Extreme Base Station Features
This software provides support for the Wi-Fi Protected Access (WPA) specification for the AirPort Extreme base station and AirPort Extreme client. Please see AirPort help for more information on WPA. Also included in this release is v5.2 of the AirPort Extreme base station firmware. Instructions for updating the base station firmware can also be found in AirPort Help.
The AirPort Extreme base station and AirPort Extreme client are Wi-Fi Certified for 802.11b and 802.11g interoperability.
You can download the update directly in Software Update Control Panel in Mac OS X. The update weighs in at 8.8 MB.