How do I keep track of the bandwidth usage through my Airport Extreme wireless router?
The Airport Express/Extreme allows monitoring via SNMP if you enable it via the Airport Configuration Utility (version 5.6 ONLY).

Apple removed advanced configuration parameters from Airport Configuration Utility 6.0. Please download a version 5.6 of the utility from Apple’s support page to get a full access to all configuration parameters of your router. SNMP configuration is located under Advanced settings.

From there, ANY snmp polling engine can get traffic as a whole.

You would be hard-pressed to find anything that could differenciate traffic between devices that isn't a real router (Cisco, etc) or custom built Linux box.

I just found this during a quick serach: NetUseApp via iTunes Store allows you to monitor any SNMP device with a pretty interface.