One of the interesting new things in iPod/iPhone OS 3.0 is the new "find my iPod" feature. It's probably most useful for an iPhone, which could be easily left on a bus or something, and has an always-on 3G connection, but it works just fine for iPods over WIFI as well.
It's close - I'm just on the edge of the blue circle as I'm typing this - but it's good enough to tell me that I haven't left it at home. The map updates in nearly realtime, so you could, in theory, track the device as it walks away.
The location is only visible to the owner of the MobileMe (nee .Mac) account, and, I suppose, The Authoritiesâ„¢. It's intended to provide a way to find a lost or stolen iPhone. It could also provide a creepy way of tracking someone, but you'd need to know their MobileMe login.
You can also send messages to the iPod from the MobileMe web interface:
The message is customizable, and you get an email after the "OK" button has been clicked to confirm that the message was received.