This story blew up on the internet last week. In brief, any iPhone running iOS 4.0 or later is maintaining a record of the cell towers it connects with, and storing that record locally on the iPhone. There's been a lot of out-cry about invasion of privacy, to the point that Senator Al Franken sent a letter to Apple for answers.
This entire issue aligns very closely with what I'm doing with this website and Google Latitude. For instance, here's a visualization a reporter put together from the data stored in his iPhone:
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The important difference here is that I knew I was being tracked. And I'm in a very small minority when it comes to acknowledging that fact.
Criticizing Apple is practically an internet sport, but it's important to keep in mind that all this data is stored locally on the phone. It cannot be accessed without physical possession of the device or the computer with the iPhone backup file. And Apple devices are not alone in this: Android devices also keep track of your location, though it appears they cull location data after a duration.
This news story should really serve as a wakeup call to consumers. If you are carrying a phone or a device with a cellular connection, you are being tracked. Even if the device itself does not maintain a record, the phone companies know every cell tower to which your device has connected. Wired connections are not any better today, as geolocation based on IP addresses is becoming extremely accurate. And that's just talking about your physical location. Think you know what web services like Google and Facebook know about you? Think again. If you have a Google account, check out your Google history sometime.
The danger here is not from data collection itself, but from people being unaware of it. If you want privacy, ditch your phone and computer and visit a library. Otherwise, recognize that some computer out there is aware of what you're doing.