As you can see, Apple released iOS 4.2 and the most noticeable new feature is free version of Find my iPhone. The function Find my iPhone (or iPod touch or iPad) can now be used for free without a subscription to MobileMe and helps users find the lost device.
Instructions :
Update your iPhone, iPod touch, or iPad to iOS 4.2 (coming later today)
Go into Settings: Mail, Contacts Calendars
Tap Add Account
Tap MobileMe
Enter your Apple ID (iTunes Store ID, Apple Online Store ID, etc.)
Verify your account via ...