Apple has released version 3.3 of the AirPort software. The new version adds support for Wi-Fi Protected Access to AirPort Extreme base stations and AirPort Extreme and AirPort clients. Also included is version 5.3 of the AirPort Extreme base station firmware. From Apple:

This software update provides improved AirPort wireless networking software, and is recommended for all users with an AirPort Extreme and AirPort enabled computer or an AirPort Extreme base station.

New AirPort Extreme Features

This software provides support for the Wi-Fi Protected Access (WPA) specification for the AirPort Extreme base station and AirPort Extreme and AirPort clients. Please see AirPort Help for more information on WPA. Also included in this release is v5.3 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.

The 8.5MB download is available from Software Update. As of this writing, the update is not yet available from Appleis site, but can be found through the Software Update Control Panel. The update is an 8.5 MB download.

If you are interested in buying an AirPort Extreme Base Station, you can find them at the starting at US$199.