You can find your device’s current location as long as:
The device is online. (If you’re looking for an iPod*touch or iPad*Wi-Fi, it can be found only if it’s on, connected to a Wi-Fi network, and not asleep.)
Find My iPhone is turned on on the device
Location Services is turned on (in Settings > General)
Note:**You can use Find My iPhone only on devices on which you’ve set up your MobileMe Paid Subscription or MobileMe Free Account. If you have more than one subscription or account, only the one you’ve set up to sync your MobileMe contacts, calendars, or bookmarks works with Find My iPhone.
This message appears if no device has been setup with your MobileMe account and Find My iPhone is turned on. If you still have the device in your possession, set up Find My iPhone in Settings > Mail, Contacts, Calendars > MobileMe.
"Your device is no longer locatable" message
If you see this message, Location Services (or Find My iPhone in iOS 3.x) was turned off on your lost device*while*you were using the app. You will not be able to locate your device anymore, but you can still display a message on its screen, have it play a sound, remotely lock it, or remotely wipe it.