That's going to be the catch cry for future supermarket checkout chicks, if indeed in the future there will be checkout chicks (self service machines are slowly taking over). Besides the obvious environmental (less trees chopped down) and economic benefits (reduced inventory costs), going electronic will actually improve the customer service experience. "Tracking" mark my words is the next big thing in the supermarket customer service experience. Customers googling their spending habits. Now it's going to be so much easier.
So how will it work? There are several possibilities.
1) Online. Once your transaction is finished, your scan your loyalty card and the reciept is automatically delivered in pdf/other format to your online supermarket account. Should you decide to print it out, you can also do that. In the future, if you lose your reciept it won't necessarily be gone forever. No more picking through dirty laundry to track down those buggers!
2) Via email. Don't have a loyalty card? Give them your email, and the cash register will send it directly to your email account.
3) Via Mobile phone. Don't trust the checkout chick to get it right? Send it to your mobile phone for instant review.
Two issues that will be the key to get this right is privacy and protecting the integrity of the data. Woolies, Coles, who'll take the lead?