Apple has recently eliminated one of the configuration requirements and potential points of failure for Find My iPhone (FMI) on your iOS device.
About a year ago we reported that there were three settings that you had to have enabled in order for FMI to work:
1. Under Settings > Mail, Contacts, Calendars > (Your Mobile Me account), you must set "Find My iPhone" to ON. This is not enabled by default.
2. Settings > General > Location Services has to be on.
3. Under Settings > Mail, Contacts, Calendars > Fetch New Data, you must either enable Push OR have fetch set to Every 15 minutes, 30 minutes or Hourly. If you disable Push and set Fetch to "Manually," Find My iPhone will not work.
The first two requirements were fairly obvious. If FMI wasn't enabled, and if Location Services were disabled, you wouldn't expect FMI to work. The third requirement was a little less obvious, and I only discovered it when I tried to set mail to "manual" and was warned that it would make FMI cease to function.