This Blog has an interesting article and link to the website of a german newspaper article (translated here).

The story is about a german politician Malte Spitz who sued to obtain the retained cell tower records for his own phone, then provided them to the newspaper. The newspaper has created a nice map and timeline tool to allow you to play Spitz's movements over 6 months. The resolution is impressive and should be a real wake up call about the level of detailed information being gathered on us all.

Of course, if the phone company was capturing GPS or WiFi based location information the data would be much more accurate. While GPS would quickly drain the battery, many modern phones have WiFi enabled all the time, so that information would be readily available without any additional impact on the phone's performance.