I'm just back from Singapore, where they have solved the "litter" problem. Single-fare rides are doled out by fare machines on plastic smart-cards. You pay a S$1 (about US$0.70) deposit for the card. After use, you reinsert the card into the machine for a S$1 refund. There are NO fare cards littering the system!
I thought NYC Transit could do a great deal for the environment by having a $0.01 value to used MetroCards. Let people collect them (there are many roming the NYC subways who have nothing else to do
and trade them in, 100 for a buck. NYC Transit gets to resuse the cards, and the homeless get to make some money.