Apple has just released a new update for AirPort Extreme base stations. The update includes general fixes, security improvements and addresses compatibility for the 802.11n base station. To install this update, you'll need AirPort Utility 5.1 for Mac or Windows.
The first security concerns seem to be the AirPort's ability to allow incoming IPv6 connections, which it does by default. The new update changes those default settings to limit inbound IPv6 connections to the local network only. You can check your settings after installing the update by opening AirPort Utility, selecting an 802.11 Extreme base station, clicking Advanced -> IPv6, and making sure that Block incoming IPv6 connections is checked.
A second security issue allows file names from a password-protected AirPort Disk to be viewed by users on the local network without a password. This is patched to require validation before file names (not file contents) can be seen.