For a few months now, your iPhone has been storing your location data and transferring it to your computer when you sync. You can view this data with the handy and open source iPhone Tracker. Getting this sort of information from cellular companies usually requires some form of legal action. But, now you can get it without ever contacting an attorney.
It reminds me of this Tell-all telephone data visualization that offers a more polished interface of one person’s travels. It also overlays some other relevant information like Call and Twitter activity.
UPDATE 2011-04-23 (via Slashdot): Apparently the data stored on the iPhone isn’t super high-fidelity.