iOS 4.2 has already launched for all supported devices. One of its coolest new features is the free access to the previously paid Find My iPhone service. Unfortunately this applies only for iPhone 4, iPad and iPod Touch 4G.
Well, there is a way to activate it on your older iDevice. You just need an iPhone 4, iPad or iPod Touch 4G and a few spare minutes.
First you need to setup sync with MobileMe account by providing your AppleID details on one of the new devices (iPhone 4, iPad, iPod Touch 4G). Then activate the Find My iPhone service, get the free Find My iPhone app from the AppStore, launch it and log in.
Once you’re done, Find My iPhone is active for your AppleID so you can go ahead and setup a MobileMe sync on the older iDevices that you want to track. Logging on with the same AppleID activates the service for those too. If the iPhone 4 you’ve used for the initial setup is not yours you can even go ahead and remove the original MobileMe account from it. Once activated to your AppleID, the service will continue to work on any device you log in to.
Unfortunately, there’s a limit. You can to setup the service on a maximum of three devices including the iPhone4/iPad you used in the initial setup. And that limit is tied not only to your AppleID but to the iPhone4 too so you can’t use it to activate MobileMe for other AppleIDs.
Once up and running, Find My iPhone is capable to find, lock or wipe the particular device or send a message to it.
If you have the Find My iPhone service enabled on your iPhone4, I strongly recommend you to turn on the Accounts and Locations restrictions. They would prevent anyone who has got your iPhone to actually disable the MobileMe sync and the Locations services, effectively disrupting the Find My iPhone service.