Geofencing, sending alerts or content only to folks within a specific location, is a very interesting idea with applications that go far beyond beaming coupons to people as they pass by your store or tracking how much time someone spends in one place.
Making content available for folks to ‘pull’ when they are in a specific place is fairly easy. Pushing content to users in a specific place is more challenging. Continuously polling for location can quickly drain any battery and polling too infrequently may make you miss a trigger point.
Location Labs has an iPhone library library in private beta that may improve upon battery performance without missing opportunities to get the right content.