I bought the program, it's actually quite slick. It starts up on bootup, validates the ID number of the SIM card against a list that you define, and if it matches the ID's in the list then the program terminates. If the SIM ID does not match, it sends out an SMS and/or email to predefined numbers and addresses. It has a test mode so that you can make sure it all works.

The application is password protected so that someone else just can't add a new SIM ID to it (say, if they know what this app does) before the next reboot. If the finder of the phone is smart enough to wipe the phone before putting in thier SIM, then of course this app will do no good (since it'd be gone), but if they do put a new SIM in before wiping the phone, then the alerts will be sent out. You can customize the alert message too.