In this week's On Language column, William Safire of the New York Times stops short of the silly "no problem"-style objection to the common "I'm good" response to a "Can I get you anything?" request (I didn't ask whether you were good or bad; I simply inquired about your potential desire for a Diet Pepsi!) but still expresses wonder at such a novel use of the word "good."
To me, the response makes perfect literal sense. Would you like a Diet Pepsi? No, thanks, I'm fine, or good, without one. I am happy with the status quo. I will inform you later if my thirst makes me less than good.