With the IOS5 and the iCloud service, it is very easy to find you lost or misplaced iPhone, iPad or Mac.
All you need is to download and update the latest IOS 5.x on your iPhone, iPad or Mac, while rebooting, it will promt whether you want to "Set up iCloud", say Yes, don't worry it won't cost you anything. Sign in with your Apple ID and follow the onscreen instructions.
Once you have completed the iCloud setup, it will prompt whether you want to sign up for "Find My iPhone.". Say "Yes", I can assure you someday it is going be very handy.