Apple just launched a Find My iPhone app in the iTunes App Store. It's a service previously that until now you could only access from MobileMe.
The service still requires a MobileMe account.
The new App will find your iPhone or iPad or iPod touch.
It has to be installed on a different iPhone, iPad, or iPod touch, than the one you are looking for. Sounds obvious, but maybe not.
An owner can log in to their MobileMe account and locate the phone in question, and then remotely lock the device or wipe the data. The App can only be works on iOS 3.1.3 or later devices and is available for free.
So, if you have someone you want to keep an eye on, make sure they install this app on their phone. Assuming of course you have their MobileMe log in information.