I was thinking about a return customer project for those programmers.
I don't have a business, so I don't personally have a use for it, but perhaps Bitpay or merchants would be interested.
Each bitcoin can be traced through the blockchain to know if it's been spent at your business before, yes?
So for bitcoins that have been "spent" at your business before, you could offer a 5% or whatever percentage you think is worth it for returning customers.
Even if the customer wasn't the one to purchase from your business, the discount wouldn't be that huge, so it'd just be a way to bring more sales.
If you could create a program that integrates with past payments and confirms the bitcoins were previously spent, that could create a value to bitcoins spent at your shop before.