I checked out both applications. For starters, neither will share your location with anyone until you explicitly agree to such sharing with each individual friend. So you can install either one and see how it looks without divulging where you are.

Also, after inviting a friend to share his or her location, or being invited to do so yourself, you can go back and change the setting to stop sharing your location with a particular friend and continue sharing with others, or stop sharing with anyone.

But what happens if you set up either app to share with friends, and forget about it? Or what if someone else puts it on your phone, without your knowledge, to track you?

In what's usually seen as a limitation, the iPhone doesn't allow running programs in the background--so Loopt can't update your location unless you open the app (Google Latitude, when it becomes available for the iPhone, should work similarly).

But most other cell phone platforms allow background processes to run silently--a potential problem. Within a few days of installing Loopt, however, you'll get an SMS notice so you'll know it's there. Loopt CEO Sam Altman also says that if you don't use Loopt for a while it will automatically stop sharing your location-likely within a week of nonuse. Google Latitude will display a pop-up notification on all phones save Android-based devices (whose users will receive an e-mail, Google says), but it won't automatically shut off.