Apple released Time Capsule and AirPort Base Station (802.11n) Firmware 7.3.1 Wednesday evening along with Time Machine and AirPort Update 1.0. The Firmware update restores AirDisk support so users can backup via Time Machine to a USB drive connected to an AirPort Extreme Base Station.
The firmware update also includes other bug and security fixes.
The update requires an AirPort Extreme (802.11n) Base Station, AirPort Update 2008-001, and Time Machine and AirPort Update 1.0. It is available via the Software Update application, or as a downloadable installer at the Apple Support Web site.