AirPort Utility 5.5.1
Although it shows up in Software Update as “AirPort Base Station Update 2010-001,” this update is for AirPort Utility 5.5.1, and includes (at least as far as Apple’s telling us) only a pair of bug fixes. The new version properly imports all settings when importing a configuration, and it also now propagates MAC address control lists when using an extended network. (Free, 10.24 MB)
However, AirPort Utility 5.5.1 is also recommended for installing the just-released Time Capsule and AirPort Extreme Base Station Firmware Update 7.5.1. That update is appearing on the Apple Support Downloads site, though not in downloadable form; see this article for information on how to install base station firmware updates. It claims to provide several fixes for the most recent Time Capsule and AirPort Extreme Base Station (Late 2009) units, including an issue that could hurt performance in the 5 GHz band and another issue with creating a guest network in the 5 GHz band.