the free Find My iPhone seems to require an iPhone 4 for setup, even though it will work on an iPhone 3GS. How can I set up the service without an iPhone 4?
The service is only free to users of the iPhone 4, and if I recall correctly users of the iPad.
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.
You could try signing up for the trial of [MobileMe], set up Find My iPhone and then cancel the subscription before the trial period expires. I don't know if the Find My iPhone feature would stick around but it is worth a try. You may find that you like MobileMe and want to keep it, if so you can save some cash by buying the subscription from [Amazon.com].
You don't need to have a full MobileMe account to use find my iPhone, just iOS 4.2. One step that they seem to leave out is that you should download the "Find My iPhone" app from the App store. I set up my iPad with it and now works.