I have a Macbook Pro with Snow Leopard, any way to do this?
Depending on your reason for turning off the Airport card (as in use the wired connection instead of the wireless when it's plugged in) you may not even need to turn off the Airport card.
If you go to the Network Preference Pane in System Preferences it's possible to set a Service Order so that when one is active it takes precedence over others. For example in the following screenshot I always use a wired connection first if it's available otherwise it would then fall back to my Airport network. My iPhone is the last on the list for the connections (Bluetooth DUN and FireWire aren't used in my case).
Maybe too much overhead, but Marco Polo is a nice application that lets you turn off/on various settings/devices depending on various factors (context aware). I have not tested it, but maybe you can create a rule that turns off the AirPort when an Ethernet cable is connected.
However, it is no longer supported by the author. As of August 2013, the author recommends looking into ControlPlane
Within the OS you can create multiple locations in Network System Preferences, one with Airport enabled and one with it disabled. In Keyboard and Mouse you can then assign keyboard shortcuts based on the location names, as they are found under Apple menu --> Location.
NetworkLocation should be able to automate this.