Apple today released Time Capsule and AirPort Extreme Base Station Firmware Update 7.5.1 which 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
• An issue with creating a Guest Network in the 5GHz band
It is recommended that AirPort Utility 5.5.1 be installed before updating to Firmware version 7.5.1.
Detailed information on Firmware Updates 7.5.1, including how to install the update, here.