iPhone 3GS how to enable or subscribe for free Find My iPhone
The service is only free to users of the iPhone 4, and if I recall correctly users of the iPad.


  It’s a feature that’s part of MobileMe, but now it’s also free on every iPhone 4 with iOS 4.2.*


From the Apple Website, bold mine

To use this functionality on any other device requires a full MobileMe subscription. It is automatically active once you have the full subscription, along with a nice array of other nice features.

The Find My iPhone feature is available when you create a MobileMe account on your device, with a toggle option to switch it on or off.

I have been using the paid subscription for almost 4 years and have been extremely happy with it due to the amount of different devices I own.