sms-sentry is an application which lets you locate your phone if it is lost or stolen. Upon sending an SMS with the text sentry:location to your phone, the phone will turn on GPS, wait for a fix and send back an SMS with the current location.
openBmap and Cellhunter are projects with the objective to collect GPS location of GSM network cells. If this project is finished then SMS-sentry could send a rough location just by identifying the current distances (strength of signal) to the available GSM network cells, even when the GPS satellites are not available (e.g. in a house). See also Anti-Theft Mode