Find My iPhone – the MobileMe service that lets users track their lost iPhone is now offered free with the latest iOS 4.2.1 update. This feature is available by default on the latest iOS models like iPhone 4, iPad or iPod Touch (4th generation).

However, if you are a user with an older model, it is possible to set up the location tracking feature on your iDevice provided you also have access to one of the newer models. 

Interestingly, Apple has explained how to enable it on older iDevices on their website (checkout footnote 3 on this page):

"You can create a free Find My iPhone account on any iPhone 4, iPad, or iPod touch (4th generation) running iOS 4.2. Once you create an account on a qualifying device, use your Apple ID and password to enable Find My iPhone on your other devices running iOS 4.2."

Here are the step-by-step instructions to set up the Find My iPhone feature on the older iPhone and iPod Touch models.

Step 1: Click here to download and install the free Find My iPhone application from the App Store on your iPhone 4, iPad or the new iPod Touch. 

Step 2: If you have already set up a MobileMe account, skip this step. Otherwise, launch the application to create a new MobileMe account. 

Step 3: From your iPhone Home Screen, navigate to Settings > Mail, Contacts, Calendars > Add a MobileMe account. Log in using your Apple ID and password.

Step 4: Once connected, turn the Find My iPhone feature on. 

Step 5: Relaunch the Find My iPhone app and login using your credentials for the set up to be complete. 

Step 6: These steps now need to be repeated for the older iDevices. Download the Find My iPhone app on your older iDevice and follow steps 3-5 to complete the set up. 

There are a few points to note here. Firstly, the Find My iPhone feature is not available in all the countries. Secondly, you should also note that Apple has restricted the number of older devices that are tied to one latest model to three.

Interestingly though, users have noted that deleting the MobileMe account in the new iPhone or iPad does not affect the set up on the older models that are tied to this device. So if you are planning to borrow your colleague's iPhone 4 to set up Find My iPhone on your older iPhone model, you can make use of this trick. 

As always, don't forget to try the feature out and let us know how it works in the comments below.