+1 and yet, most arguments will end up toward the direction of "platforms consistency," especially on Facebook-related application since they have it "OK" | "Cancel" order.
If one of your users uses 50 applications, and he uses yours once in a week, he's going to be annoyed at your application for being different; even if that "different" is supposedly better, reflexes have taught the user to scan the buttons in a certain way.
I'm not suggesting that the order we have now is better; but until one of the OSs or an application like Facebook moves to the "better" button order, I can't see why a designer might do so for his own application.