We know where you are

So it's not just me that wants to know how you determine a mobile phone's location from it's base station sign-on. Clearly there are loads of privacy issue here but maybe it's not all Orwellian. An interesting use of the technology not mentioned in this BBC article is the ability to target an SMS to a geographical location, such that when a user enters it a new text message is generated. For example, whenever I enter a particular GSM/GPRS location (as defined by the base station's geographical coverage) I receive a text message updating me on all kinds of user-specified information, such as film show times, traffic reports, even advertising. You can even leave a future message for someone that only gets deliverered when that person enters a specific location. I read about this somewhere but lost the reference so if someone else can find it please let me know.