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.
The other day when I set off to do some focused writing on my iPad, I disabled Notifications and Email push, expecting the iOS to warn me about FMI again... except that it didn't. I switched over to me.com/find on my iMac to see if I could still locate and send a message to my iPad, and indeed, both worked.
I presume that this is a feature of the recently-released iOS 4.2. Unfortunately, I don't have any devices on 4.1 or 4.0 to check.
Is this a minor feature? Perhaps, but it eliminates one-third of the possible ways that someone might fail to have FMI configured properly on their iOS device ("Find My iPhone" is a misnomer, since it will find any iOS device - well, except for the AppleTV). If you use Gmail/Google Apps for your email/calendar, you could use Google Mobile instead of push, which might help battery life a little.
Mostly, I'm just glad to see that FMI is evolving. Apple had originally made it impossible to use FMI on an iOS device (meaning that you couldn't use your iPad to find your wife's iPhone, or vice versa), but they released a Find My iPhone app a while ago. More recently, they made Find My iPhone free for anyone with current iOS hardware, which meant giving away one of the few "lock-in" features of a MobileMe subscription that couldn't be easily replaced with an offering from Google.