AirPort 4.2 is now in Software Update.
For Mac OS X:
AirPort 4.2 for Mac OS X software supports all models of Apple AirPort base stations including the AirPort Extreme and AirPort Express models.
This update fixes compatibility with Wi-Fi Protected Access (WPA2) security software on AirPort Express and AirPort Extreme base stations. This update also resolves IPv6 link local configuration issues.
For Windows:
This update fixes compatibility with Wi-Fi Protected Access (WPA2) security software on AirPort Express and AirPort Extreme base stations. It also resolves issues with WPA2 on Windows XP with Service Pack 2 and Microsoft update KB893357.
Direct downloads:
AirPort 4.2 for Mac OS X 10.4.2 (14MB)
AirPort 4.2 for Mac OS X 10.3.3 (13MB)
AirPort 4.2 for Windows (12.5MB)