If you're using an existing MobileMe account, you'll need to select the Find My iOS device service by clicking on the cloud icon in the top left corner of the screen and then selecting the icon that looks like a radar screen. If you're just using a free Apple ID, you should immediately see the Find My iOS device interface as soon as you log in.

In either case, you'll then be presented with a map of the world, along with a list of your devices running down the left hand side of the page. A green dot next to a device means that it has been located; a red dot means that it can't be found (usually because the device is powered off or in Airplane Mode). Clicking any device will show you the device's location on a map.

Once you've set up the service, you can locate your device on a map via MobileMe's Web interface.

The precision of the location--represented on the map with a blue dot surrounded by a shaded circle--depends on what method the device uses to report where it is. The GPS chip in iPhones and 3G-enabled iPads will provide the most accurate and precise locations. Unfortunately, for devices that rely on location via Wi-Fi positioning, like an iPod touch or Wi-Fi-only iPad, they'll need to actually be on a Wi-Fi network in order to report where they are.

Hovering over the device is a bubble with the name and an icon representing what kind of device it is. There's also a blue arrow: clicking on that will give you a variety of options for remotely controlling your device. Here's a rundown of what each of those abilities does: