I first learned that CO had Coke Zero on a IAH-SAN flight back in August 2008, when I saw it advertised on the napkin. The F/A gave me a cup full, and later asked if I wanted the rest of the can, which I gladly accepted. The F/A then asked if I wanted anything else, to which he told me that Coke Zero was the only can loaded onto the aircraft!
But I love the idea they are carrying Coke Zero, which I think is the best tasting Coke product.