Interesting. Still, the realtime location data Loopt have would make this much more effective. People might not want to travel to a realtime deal running somewhere in their city, in case it ends before they arrive. But if the deal is running at a bar next door or on the street they are walking along then that problem is eliminated.
How about combine this with book-in-advance?
So say you get $20 of pizza for $10, coupon website actually runs the payment system - takes payment of $10 in advance, and then you show up to get your deal. Then limited-capacity places could do realtime deals available to limited number of people to reach but not exceed their capacity, and bargain-hunters would be sure they'd get their deal and not show up somewhere that just became full.