Cell phone apps like Loopt and the new Google Latitude allow you to track your friends' physical locations, and be tracked in return. That can be a huge boon for meeting up on a Friday night-and a real nightmare for privacy if proper safeguards aren't in place. (Read more on cell phone privacy.)

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.

Google does let you limit sharing to only your city-level location, and in both apps you can enter a (possibly false) location for yourself.

Both Google and Loopt say they do not store historical locations, only your last location. That's important in case someone-the government, say, or a civil litigant-seeks that data. Loopt says it will share that info only under a wiretap or­­der. Google hasn't said it will do the same, but it does have a record of fighting government requests for its users' information.

My conclusions? Some things could be improved: First, you should be able to share your location only for a set amount of time-say, the next 2 hours, or from 6 to 9 p.m. on Fridays. Loopt says that ability will come in a future release, but Google isn't planning to announce anything along those lines.

Next, I think Google should have an auto-shutoff after a certain amount of time, in case you become forgetful. And it should explicitly declare it won't share your information without a wiretap order.

Of the two, you might try Loopt (ideally on an iPhone), since it has auto-off and will also come out with time-based controls.

But here's the kicker: As Kevin Bankston of the Electronic Frontier Foundation points out, the safeguards in place are only company policy, not a legal requirement. And policies can change.