The ads would be so engrossing that it might be hard to pay attention to the road. Change my billboard example to your GPS display or smartphone and you can see how obnoxious this might become. And this is what Foursquare and other location-based apps must protect themselves and their users against.

Making this work--for better or worse--requires a device that you have given permission to display ads in exchange for some other product or service.

A future Googlephone, for example, might be priced less for people willing to accept Google's location-based advertising. Users might even tell Google the types of ads they would like to see while out and about. You can imagine such ads, which would need to include an offer that could be clicked on for Google to get paid its full fee, would carry quite a premium price.

Foursquare and the others are just beginning our new location-based world of mobile computing. It should be exciting, fun, useful, and, hopefully, not too obnoxious.

David Coursey has been writing about technology products and companies for more than 25 years. He tweets as and may be via his Web site.