MobileMe a completely free service, but it looks like we're taking baby steps instead: Find My iPhone / iPad is now free for iPhone 4, the iPad, and the fourth-gen iPod touch running iOS 4.2.1. As with the paid feature, you'll be able to locate your device on a map, lock it, display a message, play a sound, or remotely wipe your data from any browser or the updated iOS app, which should presumably go live sometime today along with iOS 4.2.1 itself. Like we said, it's a baby step, but hey -- we never say no to free.
Update: Okay, so this is strange -- Find My iPhone is free for all devices running iOS 4.2.1, but you can only register for the service from an iPhone 4, iPad, or new iPod touch, unless you pay for MobileMe. Why? We'd guess it's to limit it to owners of the newest gear, but whatever -- if you've got an iPhone 4-toting friend you can totally get in on the action.