Apple today released "AirPort Base Station Update 2008-001" via Software Update.
The AirPort Base Station Update 2008-001 includes general fixes and compatibility updates for the following applications: AirPort Utility; AirPort Disk Utility; and AirPort Base Station Agent.
Shortly thereafter, three separate versions of "AirPort Utility 5.3.1" appeared on the Apple Downloads page.
AirPort Utility v5.3.1 is the simple to use, setup and management utility for the AirPort Express Base Station, the AirPort Extreme Base Station and Time Capsule.
Use AirPort Utility to setup and configure the following products:
AirPort Express Base Station
AirPort Extreme Base Station
AirPort Utility 5.3.1 for Leopard (8.8 MB)
AirPort Utility 5.3.1 for Tiger (9.33 MB)
AirPort Utility 5.3.1 for Windows (10.28 MB)
Update: Apple has apparently pulled the updates for unknown reasons.
Update 2: The updates are once again available through Apple's site.