Apple makes Find My iPhone feature free along with iOS 4.2 update without a tie to a MobileMe account or rather to a free limited version. As expected, from the press release:
The Find My iPhone (or iPad or iPod touch) feature is now free to use without a MobileMe℠ subscription and helps you locate your missing device.* The Find My iPhone app is a free download on the App Store℠ and lets users easily locate a missing device on a map and have it display a message or play a sound. Users can even remotely lock or wipe data from a lost device to protect privacy.
