If you don't have the paid service for MobileMe, you can get a cool feature at no cost anyway, it's the Find iPhone/iPod [Touch]/iPad service. After it's setup, you can login to your free MobileMe account and take the option to locate the device. Assuming it's powered on and on a network (wifi or cellular), you can locate it on a world map, send an audible message to the unit (great if you know it's in the house but don't know where), or remotely wipe it.
Instructions on Apple's page.
