http://periodic.phonelocator.mobi/
Phonelocator periodic is a free service to help you keep tabs on the location of your phone. It runs in the background and can periodically update the phone's location to the Phonelocator server. Updates can also be triggered remotely by sending a text message to the phone that starts with the word 'Update'. You can also nominate a friend to receive a 'buddy message' when the SIM card in your phone is changed. If your phone is stolen and the SIM card is changed, your buddy should receive a text messaged containing the thief's telephone number.
I just installed this cool (and free) app. It can run in the background and send periodic location updates to a web account, using GPS, cell or WLAN info. It will also send a text message to a designated phone when the SIM card is changed, in case of theft. A really great app considering others are charging for this service.