Now that the iPhone (iOS4) supports multitasking you can use Family Tracker, which is an iPhone app that lets you see exactly where an iPhone is. You can't do this on just any ones iPhone, you have to install it on the phone that you want to track and authorize members for each ping.
The Family Tracker app is one of the least expensive and simple ways to accomplish iPhone tracking. The app only cost $3.99 which is cheaper than mobile me or the AT&T FamilyMap. Read on for more details of how Family Tracker works.
"Install Family Tracker on the iPhone you want to track. From then on, you can use either a web browser or via the Family Tracker app itself installed on your own iPhone to locate your family member.
Using the web or your own iPhone, you will be able to “ping” the other iPhone. This is done by sending the other iPhone a (free) Apple Push Notification which will cause an alert to sound on your family members iPhone, even if the phone in is stand-by, just as would happen if the iPhone receives an SMS. As soon as this alert is acknowledged, their GPS position will be recorded, and their location will be updated on a map. If you are using an iPhone yourself to track them, you will be send a Push Notification to inform you that the family member has accepted your “ping”.
Family Tracker does not need to know your iPhone cell number to function, as it does not send SMS messages for notifications. Your email address is instead used to send free Apple Push Notifications to the iPhones being tracked. These notifications are free, specific to our Family Tracker application, and work anywhere in the world as long as the iPhone and/or iTouch being tracked has internet access via the cellular network or WiFi."