With the launch of Google Latitude, Google Maps for Mobile users can now opt into a service that allows users to see where their friends are on a map.
The Latitude service is available now on Blackberry, S60, and Windows Mobile, and is expected to be available on Android in the next few days. Plans for an iPhone version, through Google Mobile App, are in the works now. It's also available for your desktop as an iGoogle gadget.
Google has built in extensive privacy settings to protect users who don't want to be found. Users can set Latitude to show their exact location (as triangulated from cell towers, as in Google Maps for Mobile), or just the city. They can also permanently or temporarily hide from individual users or all users. And unless you opt in, no one will be able to see your location.