The previous post talked about marrying any Web 2.0 service which provides directional information to your car. Now, ignore the "appliance's" form (whether it be a car, cellphone, glasses, etc.) and consider the data.
Locations (addresses, intersections, lat/longs) could be tagged with keywords just like a website is tagged using del.icio.us. Shops could tag themselves with the products they offer, current sales, job postings.
As you walk, drive, or hop through the streets of a city, your "appliance" automatically subscribes (and unsubscribes) to data feeds based on your current position. If a feed item jives with your interests or needs, you are alerted (text message, email, cellphone call, etc.) Of course, these alerts can be micro-managed to allow for complete privacy, full-alert-onslaught, or anywhere in between.
Notice I used the word "shops" above. Here, I think you should be thinking "long tail". What shouldn't happen is huge department stores dumping their entire inventory as tags into the system. Tags should be limited in quantity and used to broadcast interesting products, sales, and shop specialties. This allows small, eclectic, out-of-the way shops to advertise their wares to consumers who are interested.
If the appliance receiving the feeds is your car and its windshield, then more cool things happen (see previous post). The magic green line leads you to a shop selling an alligator skin sofa you've been looking for. A coloured overlay causes a bookshop to "blink" a few times, with a virtual "Help Wanted" sign above it, because you're looking for a job.
Latitute-longitude pairs could also be tagged by anyone. Using my cellphone, I could tag any position that I'm at with various keywords for my group of friends or to share with the public. I could tag all of my favourite hang-outs in the city for my friends or others with similar interests to check out. You could have your cellphone autotag your geoposition at regular intervals and have a "diary" of your whereabouts during the day. Lost and found items could be tagged. Places could be tagged with historical or cultural information ("Glenn Gould used to eat breakfast here," "A battle occurred here", etc.)
The possibilities are truly endless.