Depending on your temperament, you'll have a different reaction to seeing a map of literally everywhere you've taken your iPhone (or 3G iPad) since you bought it. You might think it's a neat hack to gain some perspective on your far-ranging life. Or you might wonder why Apple has been keeping this information all along.
The application, made to prove a point of sorts, looks into the automatic backups your device makes every time it's connected to iTunes, then pulls out a lifelong list of your locations, timestamps included. No special tricks are needed, and the data is easy to get to, unless you choose to encrypt your device backups. iPhoneTracker then formats all the data for visualisation on a map, which provides a pretty neat look at everywhere you've been.
The data comes from cellular tower triangulation, regardless of whether you have GPS turned on, and whether you've allowed other applications to have access to your location. That's the main concern: very specific location data, collected without asking, available to any person (or app) that knows where to look. But if you're less concerned about Apple's intentions in collecting this data, you might simply share a map of your lifelong history location, or simply start encrypting your backups — your call.