Apple has released Airport Software 3.1.1 via Software Update. According to Apple:
This software update provides improved AirPort wireless networking software, and is recommended for all users with an AirPort Extreme enabled computer or an AirPort Extreme base station.
This software includes v 5.1.1 of the AirPort Extreme base station firmware. Instructions for updating the base station firmware can be found in AirPort Help.
This update is compliant with the final IEEE 802.11g specification for both the AirPort Extreme base station and AirPort Extreme client. Additional new features include AirPort Extreme base station and client support for Interference Robustness, which provides improved performance when 2.4GHz devices like cordless phones and wireless video distribution system cause significant interference with the AirPort network, simpler WDS setup within the AirPort Admin Utility, and more.