This script was born out of my rage of having my N900 stolen earlier this year. I wanted a simple SMS-based solution for retrieving my new N900 if it were ever stolen again. I used some python code for SMS send/recv and GPS tracking from the wiki and put it together for this.

Here's how it works:
At phone startup, the script reads the SIM card number and compares it to your own valid number. If invalid, it'll send an SMS every x seconds to a pre-defined phone number that includes IMSI and GPS coordinates. It also makes a phone call to the same number after a user-defined number of seconds (to lower chance of thief closing out the phone app).

Whether the SIM card is valid or not, the script listens for a user-defined SMS alert-code. When alert mode is activated, it returns texts to the incoming number with the same info as above (IMSI and GPS).

Installation:
1. Send thief-be-caught.tar.gz to n900
2. Extract: tar cvf thief-be-found.tar.gz -C /
3. Fill out variables at the top of /etc/event.d/banana
3. Reboot
4. Check if script is running with: ps | grep sms

Prereqs: python libraries (incl. python-location)

To close the daemon:
sh /usr/sbin/sms-quit.sh

Notes:
* Some alert code strings seem to be interpreted incorrectly by the SMS receiving procedure, so try changing the alert code if nothing happens.
* Every time the GPS coordinates are read, the internal GPS and network positioning are enabled in case they were ever disabled. The location server will also be set to supl.google.com.

I hope this helps someone get their phone back if anything unfortunate happens. I am new to sh and python scripting, so please test and improve or offer feedback.