The new iPhone software update removes the location tracking “feature”. To my knowledge, they still haven’t explained why it was there in the first place.
UPDATE (5/13): A reader points out the Apple press release. I’m still puzzled, though. The press release says:
The iPhone is not logging your location. Rather, it’s maintaining a database of Wi-Fi hotspots and cell towers around your current location . . .
But this contradicts the Guardian’s story:
The file contains the latitude and longitude of the phone’s recorded coordinates along with a timestamp . . .
I wonder who’s right. (Also, Apple has a history of collecting location information.)
Internet Scofflaw 
FYI, Apple issued a press release about a week before the update. The location info is a database of nearby wi-fi hotspots, used to speed up GPS satellite acquisition.