You could also say what I usually say to people that mention the "green" aspect of cycling.

Some studies have said that cycling is a moot point on the green front because whatever carbon footprint is removed by not driving is put back through all the machines used for manufacture and transport of food. Since cyclist tend to eat more, the footprint is just moved to some other point.

That is a sloppy example of what I say but it is because I am thinking about what I am going to eat. Im pretty hungry. Im thinking macaroni salad with tortilla chips or fruity pebbles.

Another thing I dont like hearing is, "I bet you save a lot on gas money." Being the bubble-burster I am, I usually reply, "Not really, the money I save on gas and oil goes to bike gear."