Call my car
It does get hard to be a privacy advocate when it's easy to think of interesting apps that make use of tracking infrastructure. Here's one.
How often have you wanted to talk to somebody in a car next to you on the road? Consider a system where people could register their licence plate(s) with their cell phone account. Then, if they had done this, you could call a special number on your own cell phone, and enter the numeric part of their licence plate.
If both you, and the other car were close by (for example in the same cell, but often the cell companies have much closer tracking information) and both of you were moving, it could then complete the call to the other car. The other car might get to screen the call (ie. you would have to enter the reason for the call and they would hear, "Will you accept a call from
Sounds like a good product for the cell companies, able to generate minutes. Easy enough to do if both people use the same cell company, lots more work between two different companies where a protocol would be needed. Would be easier to do with texting but you don't want people texting in cars.
Could have used it last night, was tailing a friend on the road to her house, did not have her cell number but could see her plate.
As I've described the system it's opt-in, nobody calls you unless you sign up for it and register a plate. However it could be made fairly safe to opt-in with a number of protections. As noted, the system could demand the cars are moving (cell network can see that) so that it can't be used to reach your cell phone while you are not driving. You could have screening.
It should also have a reputation system. For example, if you call me, then after we disconnect I can leave a negative reputation comment on you. Get a few of these and you're out of the system. This assures people don't use it simply to express road rage at the next driver or other things that are largely annoying. On the other hand you can use it to tell people their blinker is blinking or their trunk is open. (Mind you, once you are aware of a problem you would want a function to tell callers you are aware of a driving problem and to press 2 if they are calling about something else.)
And sure, for those open to it, it would be used for flirting with the cutie who gave you the eye when you were both stopped at the light.
You can of course just stick your cell number on your bumper to do this, but it would not have the opt-out and reptuation systems. With today's cheap phone numbers, however, you could get a special number that forwards to your cell and performs the screening/reputation/etc. but is not able to use the location awareness.
If the digits are not unambiguous (or, like me, you have a custom plate that's all letters) the system would need to offer you the cars close to you that match.