What if your product is Foursquare or Twitter? There are obvious business models down the road but only until you achieve massive adoption.
I think the question to potential users is "Would you use this everyday?". This is more important than going to Starbucks and asking if they would pay to promote within the service (it's a no-brainer).
The business model for Foursquare requires that both users and businesses find their respective propositions valuable. So essentially you have two sets of customers and you would have to ask both.
Users: would you use this everyday
Businesses: Would you pay to reach X number of your loyal customers
I think the question to potential users is "Would you use this everyday?". This is more important than going to Starbucks and asking if they would pay to promote within the service (it's a no-brainer).