While not an app, Apple's Mobile me allows for the tracking of any lost/stolen Apple Ipad/Ipod/Iphone. Once the program is setup on the device, the user can track it remotely on any computer with the ability to lock and display messages on the screen. I have been a victim of theft when someone stole my Ipad out of my room at school, and having this program would have possibly helped me catch the culprit. Setting up the program is extremely easy, directions can be found here:
http://www.apple.com/mobileme/setup/iphone_ipod.html
Also, here's a story about how MobileMe saved someone thousands of dollars in Iphones, credit cards, and other valuables:
http://www.davelongstrethmusic.com/?p=121