Look already at the flash mob deals they're doing that are location based, the deal they just struck with expedia, the deals with national brands they're starting to do more of. Those are just derivatives of their core idea but also all bring in serious cash.
But then there are things they're likely working on that will go beyond the deal space and in to other services for the tens of thousands of businesses they now have relationships with. Think cross-business loyalty cards, point of sale systems, reservation systems, etc. If you think of a small business' "stack," they're just at marketing right now with the deals stuff, but there's no reason why they can't inject themselves further on down the business stack successfully. Deals will always be a part of their business I suspect, but in 2-3 years not their core business model, just an accepted concept in a much larger offering to businesses.