AirPort Utility 5.5.1 for Mac includes general bug fixes and addresses the following issues: Not importing all settings when importing a configuration and not propagating MAC address control lists when using an extended network. The update weighs in at 10.24MB.
Apple Time Capsule & AirPort Base Station 7.5.1 provides several fixes for the latest Time Capsule and AirPort Extreme Base Station (Late 2009) including an issue with wireless performance in the 5GHz band and an issue with creating a Guest Network in the 5GHz band. Apple recommends that the above AirPort Utility 5.5.1 (Windows version here) or later be installed before updating to Firmware version 7.5.1. The update requites Mac OS X 10.5.7 or later, Windows XP SP3 or later, Windows Vista SP1 or later, or Windows 7.
Both updates can be downloaded via the links above or through Software Update.