One of the benefits of starting a MobileMe account is that it can help you locate a lost iPhone, provided you enabled the "Find My iPhone" application on your device before it goes missing. There are some restrictions to the application, however. If your iPhone has no power or is sans SIM Card, you won't be able to track it.
Tracking Without a SIM Card
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The "Find My iPhone" application needs to communicate with your wireless network to function properly. When the SIM card is taken out of an iPhone, that communication can no longer take place, and the GPS coordinates for your device cannot be established. In essence, when the SIM card is out of an iPhone it cannot be located using the MobileMe tracking service.
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Tracking Without a Battery
The battery in your iPhone is imperative to its use of the "Find My iPhone" application. If your device currently has no power, you will not be able to track it down. This is true whether the battery is physically removed, or just runs out of charge. Likewise, while you can still send a message to be displayed on your device using MobileMe, or initiate a wipe of all iPhone data, the action will not take place until power is restored.
Ease of Removing a SIM or Battery
The SIM card on an iPhone can be removed in just a few seconds; the tray that holds the card is designed to pop-out easily. A battery, on the other hand, cannot be removed from an iPhone quickly because the entire casing of the phone must be taken apart to access the battery. Bear in mind, however, that a powered down iPhone loses its tracking features just like if the battery was missing. If your iPhone is not showing up in MobileMe, it is more likely that someone has removed the SIM or turned it off than removed the battery.
Considerations
Because it is so easy to disable the "Find My Phone" feature on an iPhone, consider calling your service provider on another phone if your device does not appear in your MobileMe account. A phone that is misplaced but not in the hands of another person will most likely show up in MobileMe, so if you can't track the phone it might be a good indication that someone else has found the device and removed the SIM or battery.