Recently updated in the movement to invade your children's privacy, follow your cheating spouse, ...never lose your phone and locate your friends is Mobfindr from Navizon that will allow you to find the location of nearly any device with a text message.
How it works: The Navizon software utilizes "Virtual GPS", which uses WiFi and triangulation from cell towers to approximate your location. Mobfindr is a service that runs in the background. When an SMS containing a customized passphrase is sent to the Mobfindr-enabled device, it returns its approximate location (within about a mile) via text coordinates.
The Navizon software also features "group" and "buddy" services that allow you and your friends to locate each other.
You can try a free 15-day demo or shell out $24.99 for Navizon Premium. Mobfindr currently only works with the iPhone should be released as a separate application for WinMo and BlackBerry in the near future. A version for S60 devices also is in the works.
Peep video of the Mobfindr service after the break.