Text (8th+errata): Enchant creature. ; Whenever enchanted creature deals damage to an opponent, you may draw a card. [Oracle 2005/08/01]
You draw one card each time the enchanted creature damages the opponent. This is not one card per point of damage. [Barclay 1998/06/10]
If put on your opponent's creature, you do not draw a card when that creature damages you. The creature has to damage your opponent in order to have this work. [D'Angelo 1998/11/10]
Drawing a card is optional. If you forget, you cannot go back later and do it, even if it is something you normally do. [D'Angelo 1999/06/01]
