It's not often I'm posting computer or phone-related stuff here any more, but this one is clever and deserves wider attention:
Currently if you do not have a password on your device a thief (or child) could deactivate Find My iPhone. However most people don't know that you can prevent this by using a passcode. (This might require iOS 4.2 or later; I didn't have an earlier version to test it.)
To prevent anyone from turning it off you can do the following steps:
- Open Settings and go to 'General.'
- Scroll down and tap on 'Restrictions.'
- Turn on restrictions and enter a four digit passcode and repeat to turn on.
- Under 'Allow Changes:' turn both of the switches Location and Accounts to Off.
Now you cannot disable find my iPhone without the passcode. This also prevents changes to anything related to location services and changes made to your email accounts.
Find my iPhone is free. Works for iPads too, of course.