It looks like users with older iDevices will be able to use Find My iPhone for free as long as they know someone with a new iDevice.
According to Apple if you create your free Find My iPhone account on a new iPhone 4, iPad, or iPod touch 4G you will be able to use that account on other devices.
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. Find My iPhone is not available in all countries.
You can find instructions on how to create the account here
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Apple intentionally disabled this on the 3G and 3GS even though they meet the hardware requirements. They've done the same thing with Voice Control and other software features in the past. Apparently, this is Apple's way of creating "perks" for frequent updaters.
I know alternatives to "Find My iPhone" exist in the jailbroken realm, but I haven't tried them. I'll be impressed if a developer makes Apple's feature available on the 3G and 3GS via Cydia. It may be impossible since it requires communicating with Apple to create the free MobileMe account.