Mobile Me's Find My iPhone is a wonderful (relatively) new feature which can help track down your iPhone if it is lost or stolen, but it is not foolproof, and it must be configured before you need it.

Q: "Is it true that Find My iPhone does not work if you have 3G turned off or lose your iPhone where there is no 3G service available (EDGE only)?"

A: False. Find My iPhone works with the original iPhone, which did not even have 3G or GPS capabilities, so it does work with EDGE. If you have a 3G-capable iPhone and disable the 3G (Settings > General > Enable 3G > OFF) to save battery life, "Find My iPhone" will continue to work.

Q: "Will find my iPhone work over WiFi?"

A: True... sort of... Maybe... Not really. In my home I have very little or no AT&T service (or Sprint, or Verizon, or any other cell provider). I do have WiFi all over the house, and Find My iPhone has never failed to help me locate my iPhone when it is "lost" in my house. In order to test it purely over Wi-Fi, I put the iPhone into Airplane Mode (meaning that both EDGE and 3G were both disabled) and enabled Wi-Fi. I asked "Find My iPhone" to locate my iPhone and was told that it was near Orlando, Florida. It was, in fact, in Ohio. I repeated the test and it came back with the same information. Later, I tried the "Wi-Fi only" test from my home, and Find My iPhone could not find my location at all.

However, even in Airplane + Wi-Fi mode I was able to use the "Display a Message" and "Remote Passcode Lock" features. So you may not be able to locate it on a map, but you still may be able to connect to it.

Proper setup is your first crucial step

All of this is a moot point if you don't have three crucial settings enabled on your iPhone. Without any one of these, Find My iPhone will not work.

1. Settings > General > Location Services has to be on (this one is obvious, right?)
2. Under Settings > Mail, Contacts, Calendars > (Your Mobile Me account), you must set "Find My iPhone" to ON. This is not enabled by default.
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.