Apple has released a Find my iPhone app for iPhone, iPad and iPod touch today, bringing the MobileMe service to the three devices (it only tracks the iPhone and iPad, of course).
If your iPhone or iPad goes missing, you can install the app on any other iPhone, iPad or iPod touch and log in with your MobileMe credentials (a MobileMe account is required) to find it. Once located, the app places its location on a map. From there you can display a message on the iPhone or iPad's screen, have it play a sound (even if silenced), remote lock it or even wipe it clean.
Of course, the whole thing hinges on your having a 2nd device to find the first one. Find my iPhone is free and universal and available now.