First, create a new location called AirPort-Off in the Network preferences pane. With this new location selected, select Network Port Configurations under the Show pop-up menu, and deselect AirPort. Then select your previous location, which for most people would probably be Automatic.
Now, you can turn AirPort off by running scselect AirPort-Off and back on by running the last command, replacing AirPort-Off with the name of your normal location name. To assign a hot key to the command using a utility like Butler, create a simple AppleScript containing this one line:
do shell script "/usr/sbin/scselect AirPort-Off"
Then have Butler (or your app of choice) run that APpleScript. (With Butler, running a shell script containing only the bare command doesn't seem to work.)

