The New York Times is discussing how Apple caches and stores the location of wifi hotspots and towers in each user's iphone/ipad/ipod touch, in order to make it easier and quicker to access those points when needed. This has raised major privacy concerns. These are some of the same issues as the new smartphone technology being tested to track mileage use. However, Apple would argue that it's data only indicates approximate location, rather than GPS-specific tracking.
EPIC, the Electronic Privacy Information Center, posted changes that Apple is making in its OS4 update for these devices, to ensure that less data is stored and it is increasingly anonymized: