One cellular phone company allows friends and family to see where you are at, if you have a GPS equipped phone. This may also be a "parental" feature. There is no technological reason I know of why GPS equipped phones could not connect either a server and give their GPS location, and have custom alerts "pushed" to the phones. Every GPS phone on the market (AFAIK) has built in web. This would not be an issue, only someone coming up with the program to do it.

Of course, this will cause your location at a period of time to probably be recorded, and subject to later retrival.