As programmers we often think that users are overly sensitive about their data. What could it hurt to allow the collection of location data, for example. Here is a short video from the ACLU that might make you stop and think about persisting any data you capture.
The scenario depicted doesn't actually rely on any data captured by an app. It is enough you have your cell phone on for it to register with the various base stations as you move around. Correlate this with surveillance cameras and you have a much more accurate tracker than you might imagine.
This is more a lesson in how "innocent" data could reveal things you might not want known:
Yes it's the data mining that is really the danger.
Without the ability to process the huge amounts of data collected. the data would remain obscure. Couple location data with a few other databases listing what a building is used for plus social networking info on meetings, parties and friends and you have a very detailed picture of what someone is doing - or more accurately what they might be doing.
The American Civil Liberties Union (ACLU) says:
"Since mobile carriers are retaining location data on their customers, government officials can learn a tremendous amount of detailed personal information about you by accessing your location history from your cell phone company, ranging from which friends you're seeing to where you go to the doctor to how often you go to church. The Justice Department and most local police forces can get months' worth of this information, without you ever knowing -- and often without a warrant from a judge."
You could say that detecting potential wrong doers is a great step forward, but there are already too many cases of the over-zealous enforcement of "law" based on minor infringments. You may not have anything to hide but it is very difficult to play by the rules 100% of the time when the microscope is focused so powerfully.
If you don't like the idea of your location data data being under scrutiny, then the ACLU has a page where you can lobby your representatives in favor of the GPS Act, which seeks to control access to location data.
In this case it is difficult to see how to avoid being tracked, but the same data mining techniques could easily be used on data collected by seemingly harmless apps trying to help you make life more pleasant or easy.
Be careful what you collect and be careful what you store. If you do need to keep the data then make sure it is encrypted.