The concept is really simple: for every person that you can convince to go to a B&M (brick and mortar) such as a restaurant or store, if that person checks-in, you would charge the store. It'd work like Adsense, but in the real-world. If your B&M has a valuable offering, consumers win. B&M's win because there is no upfront cost like sending mass mailings of coupons.

What's your take on this? Would this be a good way for Foursquare to monetize? Is it a viable business model for a small startup?

He wrote a more in-depth blog post here: http://techneur.com/post/3462048907/pay-per-check-in-adsense-in-the-real-world