I've recently had an iPad stolen, and used the "Find my ____" app to try to locate it. There's also "Prey" available which does a bit more. I think there are others but I don't know any names.
But it seems to me that both are useless, because the device cannot receive a control message unless it connects to the internet, and it cannot do that unless someone who knows the passcode unlocks it and selects a WiFi AP. (Or they happen to go to a location where I have already "remembered" an SSID/password.)
Plus, unless a thief is quite stupid, won't they promptly wipe the device and treat it as a new one?