Ethernet and Airport Problems
I'm guessing here, but it your network settings may have the ethernet interface prioritized above the AirPort (this is the default setting), meaning that when both are connected it'll try to reach the internet through the ethernet connection.  You can check this easily be looking at the order the network "services" are listed in the sidebar of the Network system preference pane.  If Ethernet is prioritized above AirPort, change it by selecting Set Service Order from the action (gear) pop-up menu below the service list, then dragging AirPort above Ethernet.  Note that you need to click the Apply button for the change to take effect.