I´m not an Apple coder, so I would like to suggest an application that will help people to recover stolen iPhones:

The App should, a couple of minutes after every power-up, send an e-mail to it´s owner reporting it´s location (triangulation or GPS, if it is an iPhone 3G) and the telephone number of the currently inserted SIM card.
Obviously, the app will have to be distribuited over Cydia, since Apple´s sandbox does not allow applications to load after a power-up. The application should try to resend the e-mail up until an Internet connection is avaliable (every hour or so if it fails).
If someone codes this, I´ll surely install it on my phone...

They have applications like this around, most of them are either expensive or require a monthly fee. They also eat your battery very quickly.