there's a guy making a metal bracket that allows use of a kensington lock with the mba, but it isn't exactly a design icon...
http://snipurl.com/3ljlf
you could put it in a steel mesh bag when you aren't using it (or use your own bag and get a mesh outer for it), and lock the bag to something...
http://snipurl.com/3ljl9
...if you attach a motion-sensitive alarm to the bag (move it, LOUD noise) it will make it harder for someone to atack without detection
there are alarms you can fit to the usb port, so if someone grabs it they'll go off, but that's just letting you know someone stole it, and there's software you can install that will try to connect to a network and raise an alarm to help track it down
but if the area isn't secure, a determined thief may have the time and the tools to cut steel cables, pick/drill locks