A new version of SNMPmon has been approved by Apple.

SNMPmon is a universal app for iPhone/iPad that allows you to monitor SNMP capable devices (MacOSX, Windows, routers, printers, wifi aps ...)

The new version supports Apple's Airport MIBs.
You can get detailed information of clients connected to an Airport Express/Extreme device.

App site link: SNMPmon

iTunes link: SNMPmon