Apple will let you use its MobileMe Service for free to find your iPhone or Ipad though GPS or the Internet if you lose it. Granted the iPhone or iPad has to be on and connected to the internet. You should set this up if you have and iPhone or iPad. It will also let you have your phone send a loud sound if it is missing or wipe all the data off your phone.
Go to Settings –> Mail, Contacts, Calendars
Click on Create Free Apple ID
Fill out some information and Verify your Email Address.
Go to http://me.com, log in and find your device.