This is a great idea, but things are growing fast...
I think the best way to be sure it will work allways is to keep it very simple. I agree with all the fantastic ideas, but a very simple way to find the device must be kept. Something like:
The owner sends an SMS to the device containing (not beggining) a password (in some coutries the only safe way to do it is SMS!);
The device hides the SMS message and responds it (to the sender) with the gps position;
If the SIM card is changed the N900 sends the new number to a specific place;
That is the "must have" part. The rest of the ideas are wonderfull, but let's keep the simple way in mind...