When you sign up for the service, you must provide your iPhone's IMEI (connect your iPhone to your computer, launch iTunes, click the Summary tab for the device, and then click on Phone Number). They'll email you within 48 hours to confirm that your phone has been unlocked. You'll then have to sync with iTunes before inserting any GSM SIM card into your iPhone.
For those in the US, remember that this will only work with the AT&T iPhone, not the Verizon iPhone.
There's just one problem. As easily as someone can add your IMEI to the database to whitelist your iPhone, it can be removed as well. As a result, your iPhone will be locked again as soon as you connect it to iTunes. In other words, use this service at your own risk.