We've been playing with a script for a while and believe it's working like a charm.
So what it does is pulls random customers through the day and gives them a random discount amount when they get to checkout. It wont show until you hit the confirm order section of checkout. It wont work every time, it wont always be the same amount and it wont always be on the same product.
Go throw some goodies in your cart and see if you win! Remember... it wont show until the step before you enter your credit card or PayPal info.