Side note that on Mac OS X, buttons normally don't get the focus, so that this isn't an issue. Well except that buttons are getting the focus here. :-)  

Also since we're reinventing the wheel, please note that alternative keyboard shortcuts (e.g. command-period) don't work, and there isn't a default button set for the dialog which there should be. 

Interestingly, you can change things in System Preferences -> Keyboard -> Shortcuts so that buttons can get the focus via tabbing through the interface. If this is enabled and you tab to the Cancel button and his Return, it should still be treated as if you hit the Okay button on Mac. Hitting space while focus is on the Cancel button does treat it as if you clicked on Cancel.