[Updated Sept 3, 2013]
How can one loose a smart phone you say? It happens more often than you think. If it hasn't happened to you then count yourself lucky. Heck, even forgetting where you put the phone happens doesn't it. You can call the phone but what if you had it set on vibrate or connected to an ear piece. Here are two apps, one for Android and the other for iPhone, to help you thwart the potential lose.
Android recently introduced the Android Device Manager (ADM) service and it's FREE. It's not an app you get from the Google Play Store but a feature you can now turn on. Go to Settings / Security / Device Administrators. Tap to put a check at the Android Device Manager box. Then go to the Android Device Manager Dashboard.
The feature I most like is that you can ping the phone and it will ring at maximum volume even if the phone was set to vibrate. If the phone is lost or stolen, you can see where the phone is on a map to assist in retrieving it. Worst case, you can send a signal to wipe the phone so you data is not available to anyone else. The feature I wish it had was to lock the phone so it is unusable to anyone. For now, you can find or wipe it clean.
Apple has their Find My iPhone found in Settings / iCloud. Turn on Find My iPhone (or iPad). Then get the Find My iPhone app from the App Store and run on another iOS device if you phone is lost.
UPDATE: If you loose your iPad or iPhone go to www.icloud.com/find and log in with your Apple ID and password to discover where your devices and decide what option to take. Even make your device show a message and phone number to call so the person can return it post haste. Nice.
Above All, set up a pin code or password for your smart phone so no one can get to your apps or data if they find your phone.